When it comes to Proof of Work, computers on the network compete to solve mathematical formulas and win the right to confirm transactions into the blockchain, and thus, secure the network. The two major drawbacks to the PoW protocol is the danger of a 51% attack and the massive energy consumption required for network security.
The most widely used proof-of-work scheme is based on SHA-256 and was introduced as a part of Bitcoin. Some other hashing algorithms that are used for proof-of-work include Scrypt, Blake-256, CryptoNight.
A Proof of Work is essentially a data that is very costly to produce in terms of time and resources, but can be very simply verified by another party. The proof of work for Bitcoin is referred to as a “nonce".