Ethereum is an open-source , blockchain-based distributed contract system. It provides a decentralized virtual machine, the Ethereum Virtual Machine(EVM), which uses an international network of public nodes. Ethereum facilitates the transfer of cryptocurrency tokens called ether. An internal transaction pricing mechanism(GAS), is used to allocate resources on the network and prevent spam.
Vitalik Buterin proposed Ethereum in 2013 and managed to aquire funding mid 2014 by an online crowdsale. Ethereum was launched 30 July 2015, with the first 11.9 million coins going to the contributers of the crowdsale.
In 2016 Ethereum was forked into two blockchains, Ethereum Classic and Ethereum. This was known as The DAO hard fork
The Ethereum Cryptocurrency blockchain is called ether. It is considered one of the big 3 along side Bitcoin and LiteCoin It is listed under ETH and traded on the majority of cryptocurrency exchanges. It is also used to pay for transaction fees and computational services on the Ethereum network.
The EVM is a “runtime environment for smart contracts in Ethereum” as defined in the Ethereum Yellow Paper by Gavin Wood. It is sandboxed and completely isolated from the network, filesystem or other processes of the host computer system. Every Ethereum node in the network runs an EVM and executes the same instructions. Ethereum Virtual Machines have been written in many languages including C++, Java, Python Ruby and Go amongst others.
One of the main points Ethereum distinguishes itself from Bitcoin is it's contract system. Smart contracts are an exchange mechanisms that carry out the direct transaction of value between untrusted agents. In Ethereum, contracts are treated as autonomous scripts or applications that are stored in the blockchain for later execution by the EVM. These contracts are paid for in Ether.