Originally advocated by Back in 2002, the consensus mechanism for the two leading blockchains: Bitcoin and Ethereum. Miners compete to find a rare cryptographic hash, which is hard to find but easy to verify.  Miners are rewarded for finding the cryptographic hash and using it to add a block to the blockchain. The computing difficulty of finding the hash makes it impractical to go backward to rewrite the history of a leading blockchain. 

Proof of work (PoW) is a form of cryptographic proof in which one party (the prover) proves to others (the verifiers) that a certain amount of a specific computational effort has been expended. Verifiers can subsequently confirm this expenditure with minimal effort on their part.