Ethereum is an open source blockchain based decentralized platform that developers can use to create all kinds of different applications on. The platform was created in 2013 by, one of the most known personas in Crypto/Blockchain, Vitalik Buretin.
Ethereum utilizes smart contracts, that allow developers to use the platform in a way similar to working with a traditional coding language.
Ethereum is now the most widely used platform to develop new projects on, very large part of all ICOs have been using the Ethereum blockchain actually.