Ethereum is a decentralized platform that runs smart contracts: applications that run exactly as programmed without any possibility of downtime, fraud or third party interference.
Ethereum is how the Internet was supposed to work: it's a censorship-proof planetary scale computer, where users always stay in control of their funds and personal data.
Ethereum was crowdfunded during August 2014 by fans all around the world. It is developed by ETH/DEV with contributions from great minds across the globe. If you want to know more then check out the full Ethereum Wikipedia article.
ETH the beginning
The official ethereum community thread on bitcointalk from the very beginning.