🧱 How Blockchain Works

Last verified: April 2026

Blocks, chains, hashes, nodes, and the full process by which a transaction becomes permanent — step by step, plain English.

The problem a blockchain solves

Imagine a shared ledger — a record of who owns what — that thousands of strangers around the world all hold a copy of, with no bank or company in charge. The challenge is obvious: if anyone can write to it, how do you stop people cheating, and how does everyone agree on the same version? A blockchain is a clever answer to exactly that problem. It lets a group of mutually distrusting parties maintain a single, shared, tamper-resistant record without any central authority.

Blocks and the chain

Transactions are bundled together into "blocks." Each block contains a batch of transactions, a timestamp, and — crucially — a cryptographic fingerprint (a "hash") of the block before it. Because each block points back to its predecessor, the blocks form a chain. This linkage is what makes the ledger tamper-evident: change anything in an old block and its fingerprint changes, which breaks the link to every block after it. To rewrite history, you'd have to redo every block since — and out-race the entire network doing so.

How everyone agrees: consensus

New blocks are added through a process called consensus — the rules by which the network agrees on which block comes next without trusting any single participant. Proof of Work (used by Bitcoin) has computers compete to solve a hard puzzle; Proof of Stake (used by Ethereum) has validators put up capital as a bond. Either way, the result is the same: a new block is agreed, added to the chain, and copied to every participant, so everyone's ledger stays in sync.

Why it's hard to cheat

The security comes from decentralisation plus cryptography. Thousands of independent copies mean there's no single computer to hack or shut down. The chained hashes mean tampering is instantly detectable. And the consensus rules mean changing the record requires controlling a majority of the network's resources — enormously expensive on a large chain. This combination is what lets a blockchain be trusted without anyone being in charge.

What this enables

Once you have a shared, trustworthy ledger that no one controls, you can record more than just payments: ownership of tokens, the logic of smart contracts, digital identity, supply-chain provenance, and more. That general-purpose quality is why blockchains underpin everything from Bitcoin to DeFi to NFTs.

Public vs private blockchains

Most well-known blockchains are public and permissionless — anyone can read them, run a node, or transact, like Bitcoin and Ethereum. There are also private or permissioned blockchains used by companies and consortia, where participation is restricted. They trade openness for control and speed, and are used for things like supply-chain tracking. When people talk about "crypto," they almost always mean the public, permissionless kind, where the absence of any gatekeeper is the entire point.

A quick mental model

If it helps: picture a notebook that thousands of people each hold an identical copy of. Every few minutes a new page is added, glued to the previous one in a way that makes tampering obvious, and everyone updates their copy at once. The rules for who gets to add the next page, and how everyone checks it's honest, are the consensus mechanism. That's a blockchain — a shared notebook no single person owns or controls.