Blockchain

A blockchain is a database that many computers keep in sync without trusting each other. Transactions are bundled into blocks, each block contains a cryptographic fingerprint of the block before it, and the result is a chain stretching back to the first block. Changing an old record would change its fingerprint and break every block after it, which is what makes the history so hard to tamper with.

The network agrees on which block comes next through a consensus mechanism. Bitcoin uses proof of work, where miners compete by spending computing power. Ethereum and most newer chains use proof of stake, where validators lock up coins as a bond. Either way, no single company or government controls the ledger, and anyone can download the full history and verify it.

Public blockchains like Bitcoin and Ethereum are open to anyone. Private or permissioned blockchains restrict who can participate and are used mainly by enterprises. The technology trades efficiency for trust: a blockchain is slower and more expensive than a normal database, but it allows strangers to transact without a middleman.