What Is Blockchain and How Does It Actually Work?

What a blockchain literally is

Strip away the hype and a blockchain is exactly what it sounds like: a chain of blocks. Each block is just a batch of data, usually a list of transactions, plus some bookkeeping information. What makes it a "chain" is that every block contains a cryptographic fingerprint (called a hash) of the block before it.

A hash is the output of a function that takes any input and turns it into a fixed-length string of characters. Change even one character of the input and the hash comes out completely different. Because each block stores the previous block's hash, the blocks are linked together in order, each one pointing back at the one before it, all the way to the very first block ever created.

This is the part that matters most: if you tried to sneak a change into a block from a while back, its hash would change too. That new hash no longer matches what the next block says it should be, and the mismatch is trivial for anyone to spot. If you're still fuzzy on what actually lives inside these transactions, our guide to what cryptocurrency is covers that from the ground up.

What happens when you make a transaction

Say you send some crypto to a friend. That transaction doesn't land on the blockchain instantly. It first gets broadcast to the network and sits in what's usually called the mempool (short for "memory pool"), a holding area of transactions that haven't been confirmed yet.

From there, the computers running the network, generally called validators or miners depending on the blockchain, pick transactions out of that pool and bundle a group of them into a candidate block. They check that each transaction is actually valid (the sender has the funds, the signature is legitimate, nothing's been spent twice) before including it.

Once a block is built and accepted by the network, it gets added to the chain and every transaction inside it is considered confirmed. Depending on the blockchain, this can take anywhere from a couple of seconds to around ten minutes.

Consensus: how the network agrees on one true version

Here's the problem a blockchain has to solve that a normal database doesn't: there's no single computer in charge. Thousands of independent machines are all trying to build the next block at roughly the same time, and they need to end up with the exact same ledger, not thousands of slightly different ones.

The process that gets everyone to agree on which block comes next, and therefore on one single, shared version of the transaction history, is called consensus. Different blockchains use different rules to decide who gets to propose the next block and how the rest of the network verifies and accepts it. You'll hear terms like Proof of Work and Proof of Stake thrown around here; we go into how those specifically work in a separate article, so we won't get into the mechanics here. What matters for now is just the idea: no block gets treated as final until the network, as a whole, agrees on it.

Why this makes tampering so hard

Because every block references the hash of the one before it, changing an old transaction means recalculating that block's hash, which then breaks the link to the next block, which breaks the link to the one after that, and so on all the way to the current tip of the chain. You'd have to redo that work for every single block that came after the one you're trying to change.

On a network with real activity, thousands of validators are already extending the chain further every few seconds. To pull off a change unnoticed, you'd need to redo all that work faster than the honest majority of the network is adding new blocks, and you'd need to do it across a majority of the network at the same time. On an established blockchain like Bitcoin or Ethereum, the computing power and coordination that would take is so far beyond what any single actor realistically has that it's treated as practically impossible. That's what people mean when they call blockchains tamper-evident: not that changing history is mathematically forbidden, but that doing it undetected is astronomically expensive.

Public blockchains versus private ones

Most of what people mean when they say "blockchain" is a public blockchain: anyone can download the software, run a node, view every transaction, and (depending on the rules) help validate new blocks. Bitcoin and Ethereum both work this way. Nobody needs permission to participate or even to just look.

Private or permissioned blockchains flip that around. Only approved participants, often a group of companies or a single organization, can run nodes or write new blocks. These still use the same chain-of-blocks structure, but they trade away some of the openness in exchange for speed, privacy, or regulatory control. You'll see them used for things like supply chain tracking or interbank settlement, where the parties involved already know and trust each other to some degree.

Smart contracts: code that runs on the chain

Some blockchains, Ethereum being the most well known example, go beyond simply recording who sent what to whom. They let developers deploy smart contracts: self-executing pieces of code that live on the blockchain and run automatically when certain conditions are met, with no company or server in the middle making the call.

That capability is what makes things like DeFi (decentralized finance apps for lending, borrowing, and trading) and NFTs possible. We're not going to get into how smart contracts are actually written or what can go wrong with them here; that's a topic on its own. For now, the important thing to know is that "blockchain" and "cryptocurrency" aren't quite the same thing. A blockchain is the underlying record-keeping system, and what gets built on top of it, from simple payments to full applications, varies a lot from one chain to the next. If you want to see how these differences play out across specific networks, our guide to the major blockchains breaks down Ethereum, Solana, Bitcoin, and the rest. And if the terms "layer 1" and "layer 2" keep coming up as you read around, our layer 1 vs layer 2 guide untangles what those actually mean.