Proof of Work is a blockchain-based mechanism in which miners compete against each other to complete transactions on the network and get rewarded by confirming transactions and producing new blocks to the chain.
Proof of Work is in the works of many cryptocurrencies. Bitcoin was the first to use this algorithm to impose some limits on actions in the network. Bitcoin uses the Hashcash proof of work system. The most widely used proof-of-work scheme is based on SHA-256 and was introduced as a part of Bitcoin.