Ethereum is a major advancement for humankind. It empowers humans to form safe and secure digital agreements with each other, truly own and control their own money, and better harness the elegant and incredible nature of cryptography. It will change everything: the way we build things, create, communicate, and control our own futures.
Ethereum is an open-source, public, blockchain-based distributed computing platform featuring smart contract (scripting) functionality.
It provides a decentralized Turing-complete virtual machine, the Ethereum Virtual Machine, which can execute scripts using an international network of public nodes. Ethereum also provides a cryptocurrency token called “ether”, which can be transferred between accounts and used to compensate participant nodes for computations performed. “Gas”, an internal transaction pricing mechanism, is used to mitigate spam and allocate resources on the network. Colloquially, Ethereum also refers to a cryptocurrency associated with the Ethereum platform, although the technically correct term for the currency is Ether.
Ethereum was proposed in late 2013 by Vitalik Buterin, a cryptocurrency researcher and programmer. Development was funded by an online crowdsale that took place between July and August 2014.  The system went live on 30 July 2015, with 11.9 million coins “premined” for the crowdsale. This accounts for approximately 13 percent of the total circulating supply.
Ethereum is also good at things called Tokens (i.e. like Arcade Tokens, but worth more that anything you could win at an arcade). Properly constructed contracts or accounts can hold and send these Tokens. A Token on Ethereum is simply some computer code (a smart-contract) that allows token holders to hold and transfer their Tokens to other accounts and contracts. Tokens are generally created and issued during a crowdsale (where people trade Ether for some amount of the Token), but are also available on internet exchanges.