Ethereum is a decentralized platform that runs smart contracts: applications that run exactly as programmed without any possibility of fraud or third party interference.
Ethereum is used to build decentralized applications (dapps) on its platform. The most popular dapp built on Ethereum is CryptoKitties, a game that allows players to purchase, breed, and trade digital cats.
The native currency of the Ethereum blockchain is called ether. Ether can be used to pay for transaction fees and computational services on the Ethereum network.
The purpose of Ethereum is to provide a decentralized platform for developers to build dapps. Ethereum is also used as a cryptocurrency, with ether being the native currency of the Ethereum blockchain.