Ethereum is a decentralized open source blockchain. I.e. a platform for decentralized applications. There are many different types of blockchains out there. We chose Ethereum because it is the most decentralized blockchain in the world. Decentralization can be perceived as a metric that signifies the network's security and the distribution of power amongst its participants.
Ether (ETH) is the cryptocurrency used on the Ethereum network. Learn more here.