What is a blockchain?

A blockchain is a shared record of transactions that many computers keep at the same time. New transactions are grouped into blocks, and each block is linked to the one before it with a cryptographic fingerprint. Changing an old entry would change its fingerprint and break every link after it, which the other computers would reject.

Because every copy must agree, nobody needs to trust a single bookkeeper. The network follows fixed rules, called a consensus mechanism, to decide which new block is accepted. Bitcoin uses proof of work, in which computers compete to solve a puzzle; Ethereum uses proof of stake, in which participants lock up coins as a guarantee of honest behaviour.

Blockchains are useful where several parties who do not fully trust each other need one version of the truth: payments between countries, records of who owns a bond or a building, or tracking goods along a supply chain.

They are not magic. A blockchain only guarantees that the record has not been altered after the fact. It cannot check that what was written in the first place was true.