What is Ethereum?
Ethereum is a cryptocurrency first proposed in 2013 by Vitalik Buterin and later released in 2015. Original goals for the platform were the support for decentralized applications (DApps), and a more robust scripting language.
Unlike many cryptocurrencies, Ethereum employs smart contracts which are basically programmed contracts that can be use between any number of Ethereum users. Smart contracts can be programmed to execute automatically given certain conditions among a wide variety of other uses.
Compared to Bitcoin, Ether has a much shorter block time of ~15 seconds (compared to 10 minutes for Bitcoin). Ether block rewards from mining are variable over time but the platform is planning long-term to phase out the Proof of Work mining system for a Proof of Stake system.