What is a blockchain?
The shared ledger behind cryptocurrency, and why changing old records is so hard.
A blockchain is a record of transactions grouped into blocks, where each block contains a fingerprint (a hash) of the one before it. Copies are kept by many computers, so changing an old record would break the chain and be rejected by everyone else.
Blocks, chained by fingerprints
A hash is a fingerprint for data. Feed any text into a hash function such as SHA-256 and you get a fixed-length code. Change even one letter and the code changes completely. Each block stores the hash of the previous block, which is what "chain" means.
If someone edits an old block, its hash changes, the next block's "previous" no longer matches, and every computer checking the chain sees the break. To get away with it they'd have to redo every block after it faster than the rest of the network, which on a large network is practically impossible.
Why thousands of copies matter
A normal database lives with one company, which can edit it, lose it or be forced to change it. A public blockchain is copied across thousands of independent computers (nodes) in many countries. They follow the same rules and reject anything that breaks them, so no single party can quietly rewrite the record.
Public and private blockchains
- Public (Bitcoin, Ethereum): anyone can read the ledger, send transactions and run a node.
- Permissioned (some business systems): only approved members can take part. These are closer to a shared database between companies.
What blockchains are good and bad at
| Good at | Not good at |
|---|---|
| Records that many parties who don't trust each other must agree on | Fixing mistakes; reversing entries is deliberately hard |
| Transferring digital value without a middleman | Storing large files or private data |
| Proving something existed at a certain time | Knowing whether real-world information fed into it is true |
| Running programs whose rules nobody can secretly change (smart contracts) | Speed and cost, compared with a normal database |
The technical version
Each block has a header containing the previous block's hash, a timestamp, and a Merkle root, a single hash summarizing every transaction in the block. Changing any transaction changes the Merkle root, the block hash, and every later block's link. On Bitcoin, the block hash must also be below a target value, which is what makes producing blocks costly (proof of work). Ethereum instead chooses block proposers from staked validators (proof of stake).
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Frequently asked questions
Is blockchain the same as Bitcoin?
No. Bitcoin is one cryptocurrency that runs on its own blockchain. A blockchain is the general technique, and many cryptocurrencies and other systems use one.
Can a blockchain be changed?
Adding new blocks happens all the time. Changing old ones is what's extremely hard on a large public chain, because everyone else would reject the altered history.
What is a hash?
A short fingerprint of data produced by a hash function. The same input always gives the same hash, but even a tiny change gives a completely different one, and you can't work backwards from the hash to the data.
Where to next
Sources
Written for beginners and checked against the sources listed. Last reviewed October 26, 2026. This page is educational and is not financial, tax or legal advice. How we write and check pages.