Ethereum is an open source blockchain that has the added ability to run programs and smart contracts on the blockchain itself in a decentralized manner.
The native cryptocurrency of the Ethereum blockchain is called Ether, most commonly shortened to ETH. ETH can be subdivided into 18 decimal places and the smallest unit of ETH is called Wei which is equivalent to 0.000000000000000001 ETH.
Ethereum was conceptualized by Vitalik Buterin, a Canadian born in Russia who was enamored with Bitcoin, blockchain and decentralization but found Bitcoin lacking in capability. Vitalik eventually set out to design and code Ethereum and an initial tokensale was held between July and August 2014 with the price varying from 2000 ETH per BTC ($0.30 per ETH) to 1337 ETH per BTC ($0.40) by the conclusion of the tokensale on September 2, 2014. Ethereum finally launched on the 30th of July 2015 when the genesis block was mined and the blockchain was released to the public. ETH now trades at around $200 per ETH.
Ethereum introduced a novel concept of paying for transaction costs and work done on the blockchain by introducing the concept of Gas. Just like gas is used to power a car, Gas on the Ethereum blockchain is used to fuel transactions, computations, smart contracts et al. Gas is paid for in ETH and by introducing Gas, transaction costs are divorced from the amount being sent which was one of Bitcoin’s drawbacks. Ethereum addressed one of the drawbacks of Bitcoin, if you were sending something thru the Bitcoin network you would get variable transaction fees depending on how busy the blockchain was and the transaction fees would be deducted from the amount being sent which added some uncertainty specifically when exact amounts of Bitcoin needed to be sent. With Ethereum, you can set a gas cost per transaction that is divorced from the amount being sent in addition, Gas prevents people from filling the blockchain with useless spam since every action is paid for in ETH.
Ethereum seeks to add more functionality to blockchain by adding the functionality of a decentralized computer that can run programs, dApps and smart contracts without the need for a trusted central party.
Today there is a large development community composed of hobbyists, volunteers and corporations actively developing for Ethereum as a result a lot of infrastructure will go online this Q4 2018 and Q1 2019.
One issue with Ethereum is the lack of scaling, with the blockchain being able to handle only around 15 transactions per second. As the network scales and more functionality, dApps and infrastructure is added the current 15 tp/s will not be enough. To that end several protocol upgrades are actively being researched with the ultimate upgrade being a move over from Proof of Work to Proof of Stake and the implementation of sharding.