The proof of work protocol, or Proof of Work, is the best known and oldest consensus protocol that consists of the parts of a network successfully carrying out computationally expensive work to access the resources of that network.
Proof-of-work is one of the most widely used concepts within the crypto community, thanks to its success with Bitcoin. It is a very versatile tool that can be applied to many different scenarios.
The concept behind the Proof-of-Work protocol is simple: A party (which can be a miner, a user, a node, etc.) requests access to resources on a network. To gain this access, you must solve a complex mathematical problem. This problem is difficult and time-consuming to solve, but easy to verify by other parts of the network.
This type of proof is often used in cryptocurrency mining because it has stood the test of time; has been successfully applied by Bitcoin for more than 10 years. The biggest advantage of this method over others is that it does not require trust between the parties (or at least mitigates the need for trust).
The Proof of Work protocol relies on network participants doing computationally expensive work necessary to access their resources. In doing so, they produce a piece of data that other users can easily verify that a certain amount of computing power went into producing it.
This concept has been applied to many cryptocurrencies, such as Bitcoin and Ethereum, each of which requires its own proof-of-work algorithm.
Bitcoin, Ethereum (which is on the way to switch to Proof-of-Staking), Litecoin, and Monero are the most popular cryptocurrencies. Proof of Work is used by each and every one of them as a method of mining.