Proof of Work

Proof of work (PoW) is how Bitcoin decides who gets to write the next page of its ledger. Miners burn electricity guessing numbers (nonces) until someone finds a hash that hits the target. The winner adds the block and collects the reward.

The genius is that the work is hard to do but easy to check. To rewrite history, an attacker would have to redo all that work faster than the honest network, which gets impossibly expensive. The cost is enormous energy use, the main criticism of PoW.