Node

Node in more detail

A ledger is a list of transactions: who paid whom, and how much. In Bitcoin, independent computers called full nodes each keep a copy of the ledger and check every transaction and block against the rules. Anyone with a suitable computer and connection can run one.

Every block contains the hash of the block before it, so the blocks form a chain. Suppose someone alters a transaction in an old block. That block's hash changes, the next block now points to a hash that no longer exists, and every node sees the mismatch.