Ethereum is a decentralized blockchain platform proposed and co-founded by Vitalik Buterin. It was crowdfunded in 2014 by people from all over the world.
Ethereum is used to run smart contracts. They are apps that have little to no chance of censorship, fraud, third-party interference, or downtime.
Like bitcoin, ethereum is an open source project that is not owned or operated by one person. This means everyone can get this code and see how it works.
The difference with Bitcoin is that Vitalik Buterin is still an active member of the community, while Satoshi Nakamoto remains a mystery (and will probably remain so in the future).
This means that ethereum has some kind of ideological or philosophical leadership.
The currency that allows this system to work is called ether.
Functions as an open software platform through which people can use ether tokens to create and run applications and, more importantly, smart contracts.