Ethereum is one of the first crypto projects and blockchain that has introduced smart contracts. It follows Bitcoin with PoW mechanism but adds new cool usecases with EVM a programming. Their blockchain was used by many new crypto projects and it is especially popular with DeFi projects.
The potential of Ethereum with its smart contracts is huge - imagine it as a dapp marketplace where anything could be developed and deployed (similiar to Appstore) but there are some constraints such as irreversible mistakes and sharding. With the blockchain more utilized the transactions are more expensive and slower.