What is a blockchain?

Think of a blockchain as a ledger — a list of who sent what to whom — except that instead of one bank holding the only copy, thousands of computers each hold an identical one. When someone wants to add new transactions, the network follows an agreed procedure to decide whether they are valid and, once accepted, they are bundled into a “block” and appended to the chain.

The word “chain” is literal. Each block contains a cryptographic fingerprint of the block before it, so blocks are linked in sequence. Changing an old transaction would change that block’s fingerprint, which would break every fingerprint after it. On a large network you would need to redo an enormous amount of work and convince the majority to accept your version, which is what makes established blockchains very difficult to rewrite.

This matters because it removes the need to trust a particular company. You do not have to believe a bank’s balance is correct; you can check the ledger yourself. That is the genuine innovation. What a blockchain does not do is make the data true — if someone records a lie, the network faithfully preserves the lie. It guarantees that records are hard to alter, not that they are accurate.