Ethereum is a smart contract platform that enables people to build applications on it.
Ethereum was the first and us the biggest enabler of Defi (decentralized finance), which aims to have the same functionality as the regular financing system but without any banks.
Ehtereum is also the biggest platform that enables the creation of NFT (non fungible tokens), the most famous ones, crypto punks and bored apes for example are built upon Ethereum.
Ethereum are currently using the electricity heavy Proof of Work but will migrate to Proof of Stake early 2022, by merging the main chain with the beacon chain. This means that Ethereum will become 99,9% more energy efficient.
Ethereum is the infrastructure where web3 is built on. The difference from Bitcoin is the Ethereum enables functionality to be built on top. One Analogy is that Ethereum is internet and the applications (protocols) that are built on top of the websites