One record, thousands of copies
A blockchain is a ledger, which is an old word for a book of payments. The difference from a bank’s ledger is who holds it.
Thousands of computers, called nodes, each keep a full copy, and each checks every new payment against the rules before accepting it. A payment that spends coins the sender doesn’t have is thrown out by every node that sees it.
Payments aren’t added one at a time. They’re gathered into batches called blocks. On Bitcoin a new block arrives about every ten minutes on average; some come in a minute, some take half an hour.
Each block carries a fingerprint of the block before it. Bitcoin’s developer documentation puts it plainly: each block “stores the hash of the previous block’s header, chaining the blocks together.”
A hash is a fingerprint of data: change one character in the block and the fingerprint changes completely, which breaks the next link. That’s the chain in blockchain, and it’s why the record is hard to tamper with.
