Blockchain

The public ledger: all blocks linked in order since 2009.

Every Block has an 80-byte header. The header holds the hash of the previous block's header, a Merkle root that commits to all transactions in the block, a timestamp, the difficulty target and a nonce. Because each header contains the previous hash, the blocks form a single chain back to the Genesis block.

Nodes follow the valid chain with the most accumulated Proof of work. When two miners find blocks at almost the same time the chain briefly forks; the next block decides which branch wins, and the other block becomes stale. This is why exchanges wait for several confirmations.

To change a payment in an old block an attacker would have to redo the proof of work of that block and of every block after it, and then outpace the rest of the network. The deeper a transaction is buried, the more work protects it.