Ethereum (ETH) is a relatively new cryptocurrency that was released in 2015. It has the second largest market capitalization of all the cryptocurrencies, though it's a distant second to Bitcoin
which is about 5 times larger. Ethereum was designed on a different technology than Bitcoin called blockchain. While Bitcoin was developed as a currency, to be used like regular money, Ethereum is also a platform and programming language that can be used to build and run applications on. As such, Ethereum has different intentions.
Miners, who contribute computing power either towards the creation of new blocks or towards the processing of transactions, are rewarded with new crypto-tokens called ether. Ether can be traded and used to run applications. As a tradable instrument Ethereum has done well and has grown into the biggest altcoin on the market. Ethereum units can be bought or sold against other cryptocurrencies or against fiat currencies like the USD
or the EUR