Proof of work, usually abbreviated PoW, is the original, traditional consensus algorithm used on blockchain networks.
A proof of work is the piece of data created when miners solve a block to “prove” that they put in resource-intensive work generating a nonce in order to verify transactions and add a new block of data to the blockchain. Miners are rewarded for guessing the nonce if they are the first to come up with a correct answer. Miners are rewarded for the work they contribute to the network, which is verifying transactions—a process that prevents double-spending. Miners also benefit from transaction fees.
The nonce that miners seek to generate is a number that, when added to the block of transactions, causes this block to hash to a code with certain rare properties. The difficulty of creating these numbers by solving mathematical puzzles gets harder depending on the size on the network, but always remains easy for the network to check. In a large network, it is near impossible to attain enough computing power to control the network. Incentivization for mining helps ensure that a network remains large enough with enough competition to give it reasonable security.
Bitcoin served as the first major use-case for this type of consensus, although the concept had existed prior to Satoshi Nakamoto’s use in conjunction with cryptographic signatures, merkle chains, and P2P networks. Other early digital currencies adopted the same concept. Proof of work is specifically effective in scenarios that require trustless consensus.