Blockchain: how it works

A blockchain, or chain of blocks, is a distributed ledger in which transactions are grouped into blocks. Each block contains the cryptographic hash of the previous one, so later changing old data would require recalculating the entire chain and the agreement of most of the network. Many independent computers hold a copy of the ledger, so there is no single server that can be switched off or quietly modified.

A blockchain is a technology, not a single network. There are public networks, open to anyone, and private ones, controlled by an organization. That does not mean everything recorded on one is true: the network guarantees that a record has not been altered, but it does not check whether the data was entered honestly. It is also a mistake to think that every blockchain is anonymous, because most public networks openly show addresses and transfers.