Cryptocurrency and blockchain now sit inside ordinary financial life instead of at its fringe. Bitcoin trades through regulated exchanges and shows up in retirement accounts. Ethereum settles smart contracts worth billions of dollars a day. Governments write rules for stablecoins, and central banks study the same underlying technology to redesign how their own currencies move. None of that requires becoming a programmer, but it does require a working vocabulary, because most of the terms that come with cryptocurrency and blockchain were coined by engineers describing a system to other engineers, not to a general audience trying to follow the news. This guide breaks that vocabulary into 20 discrete explanations, each one built to stand on its own. A block, a wallet, a private key, a smart contract, a fork, a gas fee: these are specific, well-defined pieces of a system, not vague concepts that blur into one another on closer inspection. Understanding one doesn't require understanding all the others first, which is part of why the technology so often gets explained badly. Writers tend to assume a reader needs the full history of Bitcoin before a single sentence about Ethereum will make sense. It doesn't work that way. Bitcoin and Ethereum are different networks solving different problems. Proof of work and proof of stake are different ways of securing a ledger. A stablecoin and an NFT share almost nothing except that both live on a blockchain. Each entry here focuses on one term or one mechanism, states plainly what it is, and stops there. Some entries connect to others: a wallet holds a private key, a private key produces a digital signature, a signature gets bundled into a block. But each slide can be read on its own by anyone who lands on it directly from a search result or an AI-generated answer, without having read anything that came before it. The goal isn't to make anyone an expert in cryptography or monetary policy. It's to remove the confusion that keeps otherwise well-informed readers from following a subject that now touches banking, payments, art, law and government policy all at once. Blockchain is a ledger no single company or government controls A blockchain is a digital record-keeping system that many computers hold identical copies of at the same time, instead of one company or bank keeping the only copy. Each new batch of transactions is checked against the copies already in circulation, then added to the record permanently. That structure is what separates a blockchain from a bank's internal database. A bank's ledger lives on servers the bank controls, and the bank can edit it, freeze it, or shut it down. A public blockchain like Bitcoin's or Ethereum's lives on thousands of independent computers around the world, called nodes, each running the same software and each holding a full copy of every transaction the network has ever processed. To change the record, an attacker would need to convince a majority of those computers to accept a false version at the same moment, which grows harder as more independent operators join the network. The word "chain" describes how the record grows. Transactions are grouped into a block, and each block includes a cryptographic fingerprint, called a hash, of the block that came before it. That fingerprint changes completely if even one detail in an earlier block is altered, so tampering becomes visible immediately to every computer checking the chain. This is why blockchain is usually described as tamper-evident rather than tamper-proof: altering old records isn't impossible, but it's instantly detectable, which removes most of the incentive to try. Blockchain existed as an idea before Bitcoin, but Bitcoin's 2009 launch was the first time the concept ran as a live, working currency instead of a research paper. Since then, the same basic design — a shared ledger, a chain of blocks, a network of independent computers that all agree on what's true — has been adapted to track digital art ownership, shipping and supply chains, and voting records in small-scale trials. The technology itself is neutral. What changes from one blockchain to the next is who can add transactions, how the network agrees on what counts as valid, and what the ledger actually records.
Bitcoin was the first cryptocurrency, launched in January 2009 Bitcoin is the original cryptocurrency, created by a person or group using the pseudonym Satoshi Nakamoto and launched on January 3, 2009. Nakamoto's real identity has never been confirmed, and no one has ever moved the roughly one million bitcoin widely believed to sit in Nakamoto's original wallet addresses. The idea behind Bitcoin predates its launch by about three months. In October 2008, Nakamoto published a nine-page paper describing a system for electronic cash that would let two people send value to each other directly, without a bank or payment processor in between, and without either person needing to trust the other. The paper solved a problem that had stumped earlier attempts at digital cash: how to stop someone from spending the same digital coin twice. Bitcoin's answer was the blockchain itself, paired with a competitive process called mining that makes rewriting history prohibitively expensive. Bitcoin's supply is capped at 21 million coins, a limit written into the software from the start, and new bitcoin only enters circulation as a reward to the computers that secure the network. There's no central bank behind bitcoin and no company that issues it. Its value comes entirely from what buyers and sellers agree it's worth on a given day, which is why its price swings far more sharply than a traditional currency's. Bitcoin was designed to function as money, a way to pay for things, but in practice it has mostly become something else: a speculative asset that some investors hold the way they might hold gold, betting on its scarcity rather than spending it on daily purchases. Some merchants and payment processors accept bitcoin directly, and a few countries have experimented with treating it as legal tender, but transaction costs and price swings have kept it from replacing everyday currency for most people. Its more lasting influence has been as a proof of concept: it showed a currency could exist and hold real value without a government standing behind it. Ethereum is a blockchain network, proposed by programmer Vitalik Buterin in a 2013 paper and launched in July 2015, that runs software instead of just recording payments. Where Bitcoin's blockchain mainly tracks who owns how much bitcoin, Ethereum's blockchain can run entire programs, called smart contracts, that execute automatically once their conditions are met. That difference matters because it turns a blockchain into a platform other people can build on. A developer can write an application, publish it to the Ethereum network, and know it will keep running exactly as coded, without needing a company's servers to stay online or a company's terms of service to stay unchanged. Thousands of applications now run this way, covering lending, trading, insurance, gaming and digital art, collectively described as decentralized finance, or DeFi. Ethereum has its own currency, ether, often written as ETH, which pays for the computing power the network uses to run these programs. Every action on Ethereum, whether that's sending money, trading a token or executing a smart contract, costs a small amount of ether, called a gas fee, that goes to the computers processing the transaction. Ethereum originally secured its network the same way Bitcoin does, through mining, but switched entirely to a different method called proof of stake in September 2022, in an upgrade widely referred to as the Merge. The change replaced competitive, energy-intensive mining with a system where validators post cryptocurrency as collateral instead, a shift covered in more detail elsewhere in this guide. Every computer on the Ethereum network runs the same piece of software, called the Ethereum Virtual Machine, which is what actually executes a smart contract's code and produces an identical result on every node at once. That shared execution environment is why developers sometimes describe Ethereum as a world computer: no single server hosts an application, and no single company can take it offline, because the same program is running redundantly across the entire network simultaneously. Ethereum isn't the only smart contract platform. Solana, Cardano and others compete for the same developers. But it remains the largest by the value of assets built on top of it, and the one most other platforms get compared against.
A smart contract is code that carries out an agreement automatically A smart contract is a program stored on a blockchain that runs exactly as written whenever its conditions are triggered, with no company or person needed to approve or process the transaction in between. It behaves less like a legal document and more like a vending machine: insert the right input, and the contract produces the same output every time, for anyone, without exception. The term was coined by computer scientist Nick Szabo in the 1990s, well before blockchain technology existed to run one, but smart contracts became practical only after Ethereum launched in 2015 with a programming environment built specifically to support them. A simple example: a smart contract could hold funds in escrow and release them to a seller automatically the moment a buyer's payment is confirmed, with no bank or escrow company involved. In practice, smart contracts now run far more complex systems. A lending protocol's smart contract can accept a borrower's collateral, calculate interest, and liquidate that collateral automatically if its value drops too far, all without a loan officer or bank reviewing the transaction. A decentralized exchange's smart contract can hold pools of two different tokens and calculate a trade price between them the instant someone submits an order. The tradeoff is that a smart contract only does what its code says, including any mistakes in that code. Because the code is usually public and can't be quietly patched once deployed, a bug can be exploited by anyone who finds it first, and the damage is often irreversible. That's exactly what happened to an early Ethereum project called The DAO in 2016, when an attacker used a flaw in its smart contract to drain more than three million ether, an incident covered later in this guide. That history is why serious smart contract developers now pay outside auditing firms to review their code line by line before it goes live, treating a smart contract less like ordinary software and more like a bank vault that has to work correctly on the very first day it opens. Bitcoin mining rewards the fastest computer with new coins Mining is the process by which computers compete to add the next block of transactions to a blockchain like Bitcoin's, and the first to succeed earns newly created coins as a reward. It's also what makes the ledger expensive to falsify, since rewriting history would mean redoing that competitive work faster than the rest of the network combined. Here's how it works in practice. Miners collect pending transactions into a candidate block, then race to solve a mathematical puzzle that requires enormous trial and error to solve but almost no effort to verify once solved. The first miner to find a valid solution broadcasts it to the network, other computers check it within seconds, and the block gets added to the chain. That miner receives a block reward, currently 3.125 bitcoin following the most recent halving in 2024, plus any transaction fees included in that block. This design is called proof of work, because the only way to earn the right to add a block is to prove real computational effort was spent. It requires specialized hardware called ASICs, purpose-built machines that do nothing except this one calculation, and it consumes a meaningful amount of electricity — an amount the University of Cambridge's Bitcoin Electricity Consumption Index has estimated to be comparable to a mid-sized country's annual usage. That energy cost isn't a side effect; it's the point. The more expensive mining is in aggregate, the more expensive it becomes for any single actor to control enough of it to cheat. Bitcoin's difficulty adjusts automatically about every two weeks, tightening or loosening the puzzle so a new block keeps arriving roughly every 10 minutes regardless of how much total computing power has joined or left the network. Mining today is dominated by large, professional operations with access to cheap electricity, rather than individuals running a single machine at home, since a home computer's odds of winning that competition have become effectively zero as total network hash rate has grown.
Proof of stake is a method of securing a blockchain where the right to add the next block goes to a validator who has locked up cryptocurrency as collateral, rather than to whichever computer solves a computation first. Ethereum switched to this method entirely in September 2022, in an upgrade known as the Merge. Under proof of stake, a validator deposits a set amount of the network's native currency, 32 ether in Ethereum's case, into a smart contract as a stake. The network then selects validators to propose and confirm new blocks, roughly in proportion to the size of their stake, and rewards them with a small amount of newly issued currency for doing so honestly. If a validator approves a fraudulent transaction or goes offline for an extended period, the network can destroy, or "slash," part of that validator's staked funds as a penalty. The appeal of this system is efficiency. Because there's no computational race to win, proof of stake networks use a small fraction of the electricity that proof of work mining requires, since validators need ordinary servers rather than specialized mining hardware running around the clock. Ethereum's own switch to proof of stake cut its energy use by more than 99%, since securing the network no longer depends on burning through electricity to solve a puzzle. Critics of proof of stake raise a different concern: wealth concentration. Because a validator's influence scales with how much currency they've staked, large holders and staking pools can end up controlling a disproportionate share of block production, echoing the same centralization risk mining pools created for proof of work. Ethereum addresses part of this by letting anyone pool smaller amounts of ether together to reach the 32-ether staking minimum through a staking service, though that in turn concentrates influence in whichever staking services become the most popular. A crypto wallet stores your keys, not your coins A cryptocurrency wallet doesn't actually hold coins the way a physical wallet holds cash. It holds the private keys that prove ownership of coins recorded on the blockchain, and it uses those keys to authorize transactions. The coins themselves always live on the blockchain, not inside the wallet. Wallets come in two broad categories. A hot wallet is connected to the internet, usually as an app on a phone or a browser extension, and it's built for convenience: quick access to send or receive funds, or to interact with an application like a decentralized exchange. A cold wallet keeps its private keys offline entirely, often on a small physical device that connects to a computer only briefly to approve a specific transaction, which makes it far harder for a remote hacker to reach. The tradeoff between the two is straightforward. Hot wallets are easier to use and more exposed to online theft, while cold wallets are more secure and less convenient for frequent transactions. Many crypto holders use both, keeping a small amount in a hot wallet for regular use and the bulk of their holdings in cold storage. A separate distinction matters just as much: custodial versus non-custodial wallets. A custodial wallet, the kind most exchanges provide by default, means the exchange holds the private keys on the user's behalf, similar to how a bank holds funds in an account. A non-custodial wallet means the user alone holds the keys, with no company able to freeze the account or restore access if the keys are lost. This is the basis for a common phrase in the crypto community, "not your keys, not your coins," meaning funds held in a custodial account depend entirely on that company's solvency and honesty, a lesson driven home when several major exchanges collapsed in 2022, freezing customer funds that turned out to belong to the exchange's creditors rather than to the customers who thought they owned them.
A seed phrase is the master key to a crypto wallet A seed phrase, also called a recovery phrase, is a sequence of 12 or 24 ordinary words that can regenerate every private key in a crypto wallet. Anyone who has that sequence of words can restore full access to the wallet's funds on any compatible device, and anyone else who obtains it can do the same thing without permission. The words come from a standardized list of 2,048 possible terms, and the specific order in which a wallet generates them mathematically encodes the wallet's private keys. This matters because a seed phrase is portable in a way a bank password isn't: it can be written on paper, engraved on metal, or memorized, and it will still work to restore a wallet on entirely different software or hardware years later, with no company involved in the process at all. That portability is also the risk. There's no password reset for a seed phrase and no customer service line to call if it's lost, because no company holds a backup copy. If the words are lost, the funds tied to them are gone permanently. If the words are seen or copied by someone else, through a photo, a hack, or a phishing scam that tricks a user into typing them into a fake website, that person can move the funds out instantly, and the transaction can't be reversed once it's confirmed on the blockchain. Security guidance across the crypto industry is consistent on this point: a seed phrase should never be typed into a website, sent in a message, or stored as a photo or a cloud file, since any of those creates a copy that could be intercepted. The recommended practice is a physical, offline copy kept in a secure location, sometimes duplicated in a second location in case of fire or theft, treated with the same seriousness as the title to a house rather than as an ordinary password. A digital signature is a piece of cryptographic proof, generated using a private key, that shows a specific transaction was authorized by whoever holds that key, without ever exposing the key itself. It's what a crypto network checks before adding a transaction to a block, in place of a handwritten signature or a bank's internal verification. Every crypto wallet is built around a matched pair of keys: a private key, which stays secret, and a public key, which can be shared openly and is used to derive the wallet's public address. When a transaction is created, the wallet's software uses the private key to produce a signature mathematically tied to that exact transaction's details. Anyone on the network can then use the corresponding public key to confirm the signature is valid, without ever needing to see the private key that created it. This system solves a problem that predates cryptocurrency: how do you prove you sent a message, or authorized a payment, over a network where anyone could claim to be anyone? Public-key cryptography, the mathematical foundation behind digital signatures, was developed in the 1970s for general computer security purposes, well before Bitcoin existed. Bitcoin was simply the first system to apply it to money at scale. The practical upshot for a wallet holder is that a transaction can't be forged by someone who doesn't have the private key, and a completed transaction can't be denied later by the person who signed it, since the signature itself is permanent proof tied to that specific transfer. This is also why losing a private key is unrecoverable. Without it, there's no way to generate a valid signature for that wallet's funds, and the network has no separate identity check, like a photo ID or a security question, that could substitute for it.
A block bundles transactions and links to the one before it A block is a batch of verified transactions grouped together and added to a blockchain as a single unit, each one permanently linked to the block that came before it. That link is what turns a list of blocks into a chain, and it's also what makes the whole system resistant to tampering. Every block contains a few consistent pieces: the list of transactions themselves, a timestamp marking when the block was created, and a cryptographic hash, a fixed-length string of characters generated from the block's contents, of the previous block in the chain. Most blockchains also compress every transaction in a block into a single summary value called a merkle root, so a computer can confirm a specific transaction is included in a block without downloading every other transaction alongside it. Change even a single character of data in an old block and its hash changes completely, which breaks the link to every block that came after it. Every computer on the network would immediately see the chain no longer matches, which is why altering historical data on a live blockchain is considered effectively infeasible rather than merely difficult. Blocks are added at different speeds depending on the network. Bitcoin produces a new block roughly every 10 minutes, a pace intentionally built into the system to give a global network of computers time to reach agreement before the next block arrives. Ethereum, by contrast, produces a new block roughly every 12 seconds, a faster pace made possible by its different, proof of stake method of choosing who adds each block. A block also has a size limit, which caps how many transactions it can include. When more transactions are waiting than a block can hold, they queue up, and users can pay a higher fee to have their transaction prioritized into the next available block ahead of others still waiting. This queuing behavior is the direct cause of the fee spikes both Bitcoin and Ethereum have experienced during periods of heavy network use. Decentralization means no single point of control or failure Decentralization, in the context of blockchain, means no single company, government or individual controls the network, and no single computer's failure can take the system down. Thousands of independent computers, run by unrelated people and organizations around the world, each hold a full copy of the ledger and each have to agree before a new transaction becomes official. This is different from how most digital services work. A social media platform, a bank's online portal, or a cloud storage service all run on servers one company owns, which means that company can change the rules, shut down the service, freeze an account, or go out of business, and users have no way around that decision. A decentralized network removes that single point of control. Even if some computers running the network go offline, or a government orders a company within its borders to stop participating, the network keeps running on every other computer still connected to it. Decentralization is a matter of degree rather than an on-or-off switch. Bitcoin and Ethereum are both widely described as decentralized, but each has areas where control is more concentrated than the ideal suggests. A small number of large mining or staking operations can account for a significant share of block production, and a small number of software development teams write the code most of the network runs. Genuine decentralization is measured by how many independent parties would need to collude for the network to be manipulated, not by a simple label. The tradeoff for this resilience is speed and flexibility. A centralized database can be updated instantly by whoever controls it, while a decentralized network needs a majority of independent participants to agree before anything changes, which is slower by design. That slowness is treated as a feature rather than a flaw, since it's the friction that stops any single participant from making unilateral changes to a shared record everyone else depends on.
A stablecoin is pegged to something stable, usually the U.S. dollar A stablecoin is a cryptocurrency designed to hold a steady value, typically one U.S. dollar, instead of fluctuating the way Bitcoin or Ethereum does. It exists to give crypto users a way to hold value on a blockchain without exposure to the price swings that make most other cryptocurrencies risky to hold for everyday transactions. The two largest stablecoins by a wide margin are Tether, known by its ticker USDT, and USD Coin, known as USDC, issued by the company Circle. Together they account for roughly 90% of all stablecoin value in circulation. Both aim to maintain their peg the same basic way: for every token in circulation, the issuer holds roughly an equivalent amount of cash or short-term government debt in reserve, and allows large holders to redeem tokens for real dollars directly with the issuer. Not every stablecoin works this way. Some, called algorithmic stablecoins, try to hold their peg through code and market incentives instead of cash reserves, adjusting supply automatically to push the price back toward one dollar. This approach failed dramatically in May 2022, when the algorithmic stablecoin TerraUSD lost its peg entirely within days and collapsed to a fraction of a cent, wiping out tens of billions of dollars in value and helping trigger a wider crash across crypto markets that year. Stablecoins now function as the main way money moves inside crypto trading and decentralized finance, since traders use them to shift value between different cryptocurrencies without converting back to a traditional bank account each time. They've also become a practical tool for cross-border payments and remittances in countries with limited access to dollars or unstable local currencies, since sending a stablecoin requires only a crypto wallet and an internet connection rather than a bank account or a money transfer service. Regulation has caught up to that role. The U.S. passed its first federal framework specifically for stablecoin issuers, the GENIUS Act, in July 2025, requiring full reserve backing and regular audits from companies that issue them. Tether, the oldest and largest stablecoin, launched in 2014, years before most regulators had settled on a name for this category of cryptocurrency, let alone written rules for it. An NFT proves unique ownership of one specific digital item An NFT, short for non-fungible token, is a unit recorded on a blockchain that represents ownership of one specific, unique item rather than an interchangeable unit of currency. The word "fungible" describes something where every unit is identical and interchangeable, the way one dollar bill is worth exactly the same as any other dollar bill. An NFT is the opposite: each one is distinct, tracked individually on the blockchain, and can't be swapped one-for-one with another. Most NFTs run on Ethereum, using a technical standard called ERC-721 introduced in 2018, though other blockchains now support similar formats. The token itself usually points to a piece of digital art, a collectible, a piece of music, or increasingly a real-world claim like an event ticket or a property deed, and the blockchain record establishes who currently owns that specific token, when it was created, and its full history of previous owners. NFTs became widely known through digital art and collectible projects like CryptoPunks and the Bored Ape Yacht Club, which sold for large sums during a boom in 2021, drawing both mainstream attention and considerable criticism. Prices for most NFT collections fell sharply after that peak, and trading volume across the category has stayed well below its 2021 levels since. Beyond art, NFTs have found narrower but steadier use as event tickets that can't be counterfeited, in-game items players can verifiably own and trade across compatible games, and membership passes that unlock access to a community or a service. Some NFT marketplaces also let a creator collect a royalty automatically, coded directly into the token, every time that item resells on the secondary market, something a physical collectible or a traditional digital file can't do on its own. A common misconception is that buying an NFT of an image grants copyright over that image. In most cases it doesn't. Owning the NFT typically means owning a token that points to a file and records who owns that particular token, not the underlying legal rights to reproduce or license the artwork, which usually remain with the original creator unless a sale explicitly transfers them. That distinction has caused considerable confusion, and occasional litigation, since NFTs first became popular.
DeFi recreates financial services without a bank in the middle DeFi, short for decentralized finance, is a set of financial applications built on blockchains, mostly Ethereum, that let people lend, borrow, trade and earn interest without going through a bank, broker or other traditional financial institution. A smart contract does the job a loan officer or a trading desk would normally do, executing automatically according to rules anyone can inspect in advance. A typical DeFi lending platform lets one user deposit cryptocurrency to earn interest, while another user borrows against collateral they've deposited, with the smart contract setting interest rates and handling repayment and liquidation automatically. A decentralized exchange lets users trade one token for another directly from their own wallet, using pools of funds supplied by other users instead of a company matching buyers with sellers. The appeal of DeFi is access. Anyone with an internet connection and a crypto wallet can use these applications, with no application form, credit check or approval process, and no geographic restriction tied to where a bank happens to operate. The tradeoff is that DeFi offers none of the protections traditional finance provides. There's no deposit insurance if a lending protocol's smart contract has a bug, no customer service line if a trade goes wrong, and no regulator standing behind the platform if it turns out to be poorly built or outright fraudulent. DeFi has suffered significant losses from smart contract exploits and outright scams since it emerged around 2018 and 2019, with hundreds of millions of dollars lost across various protocols in individual incidents over the years. Because DeFi code is usually open for anyone to inspect, security researchers and auditing firms have become a standard part of the ecosystem, reviewing a protocol's code before it launches. A clean audit reduces risk without eliminating it, and users are generally expected to accept that a DeFi platform could fail entirely. A centralized exchange, such as Coinbase or Kraken, works the way a traditional brokerage does. The exchange holds custody of users' funds, matches buy and sell orders on its own internal systems, and requires identity verification before opening an account. A decentralized exchange, such as Uniswap, has no company holding custody of anyone's funds at all. Trades happen directly from each user's own wallet through a smart contract, with no identity check required. Centralized exchanges tend to offer faster trading, customer support, and a simpler experience for beginners, since the company handles the technical complexity behind the scenes. They also require what's called KYC, or know your customer, verification, typically a government ID and sometimes proof of address, to comply with anti-money laundering laws in the countries where they operate. In exchange for that convenience, users give up direct control of their funds while those funds sit on the exchange, which is why the collapse of an exchange can trap customer funds along with it. Decentralized exchanges remove that custody risk since funds never leave a user's own wallet until the moment of a trade, but they shift the burden of security entirely onto the user. There's no customer support number to call if a trade is submitted incorrectly, no way to reverse a transaction sent to the wrong address, and no company standing behind the platform if a smart contract has a flaw that gets exploited. Trading on a decentralized exchange also usually costs a network gas fee for every transaction, on top of any trading fee the protocol charges, since each trade is processed directly on the blockchain rather than as an entry in a company's internal database. That makes centralized exchanges generally cheaper for small, frequent trades, while decentralized exchanges appeal more to users who prioritize direct control over their funds and access to tokens that haven't been listed on a centralized platform yet.
Gas fees pay for the computing power behind every transaction A gas fee is the cost paid to a blockchain network, most commonly Ethereum, to process a transaction or execute a smart contract. The name comes from the analogy of fuel: every action on the network consumes a certain amount of computational "gas," and the fee is what a user pays to cover that consumption. On Ethereum, gas fees are paid in fractions of ether called gwei, and the total cost of a transaction depends on two things: how much computational work the transaction requires, and how much demand there is for block space at that moment. A simple transfer of ether from one wallet to another requires relatively little gas. A complex interaction with a DeFi protocol, involving multiple steps executed in a single transaction, requires much more, since each step adds to the total computation the network has to perform and verify. Gas fees rise and fall with network congestion, the same way a toll road gets more expensive during rush hour if pricing is allowed to float. When many people want their transactions processed at once, they compete by offering higher fees, and validators prioritize the transactions offering the most gas per unit of computation, leaving lower-fee transactions to wait. During periods of intense demand, such as a popular NFT launch, Ethereum gas fees have spiked to tens or even hundreds of dollars for a single transaction. This cost is one of the main reasons developers built what are called Layer 2 networks, separate systems that process transactions more cheaply and then settle a summary of that activity back onto Ethereum's main chain, combining most of Ethereum's security with a small fraction of its cost. Bitcoin has its own version of this cost, called a transaction fee, priced in satoshis rather than gwei, though Bitcoin's simpler transaction structure generally keeps its fees lower and less volatile than Ethereum's during periods of heavy use. A 51% attack happens when one party controls most of a network's power A 51% attack occurs when a single miner or group controls more than half of a blockchain's total mining power, or in a proof of stake network, more than half of the total staked currency, giving that party enough control to rewrite recent transaction history. It doesn't let an attacker steal coins from other people's wallets directly, but it does let them reverse their own recent transactions, a technique called double-spending. In practice, a 51% attack typically works like this: the attacker sends cryptocurrency to an exchange, waits for the exchange to credit the deposit and allow it to be traded or withdrawn, then uses majority control to rewrite the blockchain's history so the original deposit transaction never happened. The exchange is left having credited funds that, according to the now-dominant version of the chain, were never actually sent. This has happened to smaller networks in practice. Bitcoin Gold, a fork of Bitcoin, suffered an attack in May 2018 that allowed roughly $18 million worth of double-spending. Ethereum Classic, a smaller network descended from an early Ethereum split, was hit twice: once in January 2019, with about $1.1 million double-spent, and again in August 2020, with about $5.6 million double-spent. Bitcoin itself has never suffered a successful 51% attack, largely because its total mining power is now so large that acquiring a majority share would require an amount of specialized hardware and electricity that would cost far more than any plausible gain from the attack. This is why network size functions as a direct security measure in proof of work systems. The more total computing power securing a blockchain, the more expensive a 51% attack becomes, and the safer the network is by extension. It's also why exchanges typically wait for multiple block confirmations before crediting a large deposit, since a deeper transaction history is harder and more expensive for an attacker to rewrite.
A fork is a change to a blockchain's underlying software rules, and it comes in two forms: a soft fork, which tightens the rules in a way that's backward-compatible with older software, and a hard fork, which changes the rules in a way that isn't backward-compatible and can split the network into two separate blockchains if not everyone upgrades together. A soft fork works because it only makes the rules stricter, not looser. Software still running the old rules will still recognize every new block as valid, even though it doesn't understand the new restriction, so the network stays unified. Bitcoin's SegWit upgrade, activated in August 2017, is a well-known example. It changed how transaction data was structured to increase the number of transactions that fit in each block, without requiring every node to upgrade immediately. A hard fork is more disruptive because it loosens or fundamentally changes a rule in a way older software will reject as invalid. If the community doesn't agree unanimously to adopt it, the blockchain splits into two separate chains that share an identical history up to the moment of the fork, then diverge permanently from there. Bitcoin Cash split from Bitcoin this way in August 2017, after a long-running disagreement over how large Bitcoin's blocks should be allowed to grow. Ethereum and Ethereum Classic split the same way in July 2016, after the Ethereum community disagreed over whether to reverse the effects of a smart contract hack by rewriting the ledger's history. Both examples show a hard fork isn't just a technical event. It's usually the visible result of a disagreement within a community about a blockchain's core values, whether that's transaction capacity, the sanctity of an unaltered ledger, or something else entirely. Each resulting chain keeps its own name, its own currency and its own community afterward, and neither one has any special claim to being the "real" version beyond whichever attracts more users, developers and market value over time. A DAO is an organization run by code and token-holder votes A DAO, short for decentralized autonomous organization, is a group coordinated through blockchain-based rules and smart contracts instead of a traditional corporate structure with executives and a board. Decisions, such as how to spend a shared treasury or what changes to make to a protocol, get proposed and voted on by anyone holding the DAO's governance tokens, with the results executed automatically by code rather than approved by a manager. The concept's most famous early example, known simply as The DAO, launched on Ethereum in 2016 as an investor-directed venture fund and raised about $150 million worth of ether, an enormous sum for a project barely a year into Ethereum's existence. Within months, an attacker exploited a flaw in The DAO's smart contract code to drain more than three million ether, worth roughly $50 million to $60 million at the time. The Ethereum community's disputed response, a hard fork that reversed the theft, created the permanent split between Ethereum and Ethereum Classic described elsewhere in this guide. Modern DAOs are more commonly used to govern existing DeFi protocols and other crypto projects rather than to pool investor capital the way The DAO did. A protocol like a lending platform or a decentralized exchange might issue a governance token to its users, then let those token holders vote on proposals such as adjusting fees, approving new features, or allocating funds from the protocol's treasury to development grants. DAOs face practical limits that traditional companies don't. Voter turnout is often low, meaning a small number of large token holders can end up deciding outcomes that affect a much larger community. Legal status is also unsettled in most jurisdictions, since a DAO has no formal corporate registration by default, which raises open questions about who bears legal liability if the organization's actions cause harm or violate the law.