Proof of Work and Proof of Stake are two different consensus mechanisms used to confirm transactions on a blockchain network.
Proof of Work (PoW) was the algorithm made famous by Bitcoin and was previously used by Ethereum to verify transactions without third party interference. Put simply, this requires computers (known as miners) to compete against each other to verify transactions. This process requires the use of significant amounts of computing power and is therefore costly both financially and environmentally.
Proof of Stake (PoS) is the current model used for decentralised finance by Ethereum, which relieves the need for such high usage of electricity. This method uses an alternative method to verify transactions and reach consensus. While PoW mechanisms reward miners for solving complex mathematical equations in order to verify transactions and create new blocks, Proof of Stake systems select a user to create the new blocks instead.