Ethereum is a decentralised blockchain platform founded in 2014 by Vitalik Buterin. Like Bitcoin, Ethereum is an open-source project that is not owned or operated by a single individual. This means that anyone, anywhere can download the software and begin interacting with the network.
Unlike the Bitcoin network, the primary purpose of Ethereum is not to act as a form of currency, but to allow those interacting with the Ethereum Network to make and operate 'smart contracts' without having to trust each other or use a middleman. Smart contracts are applications that run exactly as programmed without any possibility of downtime, censorship, fraud, or third party interference - a smart contract will work exactly the same every time it is used.
Ethereum uses a 'virtual machine' to achieve all this, which is like a giant, global computer made up of many individual computers running the Ethereum software. The virtual currency unit that allows this system to work is called ether. People interact with the Etherum network by using ether to pay the network to execute smart contracts.
Ethereum aims to take the decentralisation, security, and openness afforded by blockchains and extend those to virtually anything that can be computed.