Ethereum is a decentralised infrastructure for computational power. Similar to Bitcoin, it connects computers worldwide and gives a blockchain network to run smart networks. You can think of it as CPU for the computer.The good thing about Ethereum is, smart contracts and the best thing about is other cryptos are using these smart contract.
The main advantage of Ethereum is the first successful Smart contract in cryto world. It is like bitcoin as the pioneer of all cryptocurrency while Ethereum is the pioneer of all Smart Contract.