What is a blockchain? A plain English guide to how it works

Most of the records that run your life live in a database somebody else controls. Your bank keeps your balance, a county office keeps your property deed, and a social network keeps your posts. So what is a blockchain? It is a different way of keeping a record: instead of one organization holding the master copy, thousands of computers each hold a copy and follow shared rules to agree on every change. This guide covers how it works step by step, and where it is genuinely useful versus where a regular database does the job better.

What is a blockchain, in one paragraph

A blockchain is a shared, append only ledger. "Shared" means many independent computers keep identical copies. "Append only" means new entries are added to the end, and old entries are extremely difficult to change once accepted. Entries are grouped into blocks, and each block is cryptographically linked to the one before it, forming a chain. Put together, you get a record that nobody owns outright and that anyone running the software can verify for themselves.

That is the core idea behind Bitcoin, Ethereum and most other cryptocurrencies.

How a blockchain works, step by step

Let's follow a single payment through a typical public blockchain.

1. Someone creates a transaction

Say Maria wants to send some coins to Sam. Her wallet app builds a message that says, in effect, "move this amount from Maria's address to Sam's address." The wallet then signs that message with Maria's private key. The signature proves the message came from whoever controls that address, without revealing the key itself. Our guide to crypto wallets explains keys and addresses in more detail.

2. The transaction is broadcast

The signed transaction is sent to the network, where computers called nodes pass it along to one another. Each node checks that the signature is valid and that Maria actually has the funds. Invalid transactions get dropped.

3. Transactions are bundled into a block

Valid transactions wait in a pool until a participant bundles a batch of them into a new block. Who gets to do that, and how, depends on the blockchain's consensus method, which we cover below.

4. The block is linked to the chain

Every block includes a hash of the previous block. A hash is a short digital fingerprint produced by running data through a mathematical function. Change even one character of the input and the fingerprint changes completely.

Because each block carries the fingerprint of the one before it, altering an old transaction would change that block's hash, which would break the link to the next block, and the next, all the way to the present. Anyone checking the chain would spot the mismatch immediately.

5. The network accepts the new block

Nodes verify the new block against the rules. If it checks out, they add it to their copy of the ledger. Sam's wallet now shows the incoming payment, and it becomes harder to reverse with every block added after it.

Consensus: how strangers agree

The hard problem a blockchain solves is agreement. With no central referee, how do thousands of computers that do not trust each other settle on one version of history? The answer is a consensus mechanism. The two best known are these.

Proof of work

Used by Bitcoin. Participants called miners compete to solve a computational puzzle. The winner adds the next block and earns a reward. Rewriting history would mean redoing that work faster than the rest of the network combined, which is prohibitively expensive on a large chain. The tradeoff is heavy electricity use.

Proof of stake

Used by Ethereum since its 2022 upgrade known as The Merge. Participants called validators lock up the network's coin as collateral. They take turns proposing and checking blocks, and they can lose part of that collateral if they break the rules. It uses far less energy than proof of work but relies on a different set of economic assumptions.

There are other approaches too, including systems where a small, approved group of companies validate blocks. Those are usually called permissioned blockchains.

Public versus private blockchains

Not every blockchain is open to the world.

• Public blockchains such as Bitcoin and Ethereum let anyone read the ledger, send transactions and, in principle, help secure the network. Transparency is high, and so is the difficulty of censoring transactions.
• Private or permissioned blockchains limit who can participate. A group of banks or shipping firms might run one to share records among themselves. They trade openness for control and speed.

A fair criticism of many private blockchain projects is that a well designed shared database, run by the same group, could often do the same job more simply.

What blockchains are good at

Blockchains shine when several parties need a shared record and do not want to trust one of them to keep it. Real uses include:

• Digital money. Sending value directly between people without a bank, which is Bitcoin's original purpose.
• Stablecoins. Tokens designed to track the dollar that move on blockchains around the clock. Our stablecoins explainer covers how they hold their value.
• Programmable agreements. Smart contracts on platforms like Ethereum run code that executes automatically when conditions are met.
• Public audit trails. Because the ledger is visible and hard to edit, it can serve as a tamper evident log.

What blockchains are bad at

The same design choices create real limits:

• Speed and cost. Every node repeating the same work is slower and pricier than one company's server.
• Garbage in, garbage out. A blockchain can prove a record was not changed. It cannot prove the record was true when entered.
• Privacy. Public ledgers are visible to everyone. Addresses are not names, but activity can often be traced back to a person.
• No undo button. Mistakes and thefts are usually permanent. There is no customer service desk.
• Human weak points. Most losses come from scams, stolen keys and buggy apps, not from the chain itself being broken.

How to judge a blockchain claim

When a company says it is "using blockchain," a few questions cut through the hype:

1. Who runs the nodes? If it is only the company itself, it is effectively a private database.
2. What problem does shared control solve here that a normal database would not?
3. How does data get onto the chain, and who checks it is accurate?
4. What happens if a user loses their key or makes a mistake?

Good answers suggest a real use. Vague ones suggest marketing. You can find more explainers like this in our crypto section.

Frequently asked questions

Is a blockchain the same as Bitcoin?

No. Bitcoin is one application that runs on its own blockchain. The blockchain is the underlying record keeping technology, and many other networks use their own versions of it.

Can a blockchain be hacked?

Large public blockchains are very hard to rewrite because doing so would require controlling a huge share of the network's computing power or staked coins. Most crypto losses come from attacks on exchanges, apps, smart contracts or individual users rather than the chain itself.

Who controls a public blockchain?

No single party. The software's rules are enforced by the people running nodes, and changes generally require broad agreement among developers, node operators and users.

Why do blockchains use so much energy?

Proof of work chains like Bitcoin use a lot of electricity because miners compete by running hardware nonstop. Proof of stake chains use far less, since they rely on locked collateral rather than computing races.