Ethereum is a decentralized operating platform. In practice, it introduced the concept of "decentralized apps". Vitalik Buterin is the platform's founder and has been involved in the development of Bitcoin and blockchain since its inception.
Many cryptocurrencies today are being issued through ICOs that use Ethereum. These tokens, known as ERC-20 are used to run scripts in the Ethereum Virtual Machine. Ethereum developed "smart contracts" which allowed tokens to be signed that had a specific function. This also made it possible for blockchain "transactions" to be expanded.
The Ether is its key cryptocurrency, and is used to make payments in smart contracts or dApps. GAS is a cryptocurrency that Ethereum uses to fuel operations that require computation from its nodes. GAS prices are proportional to the amount of computational power needed to complete a task. To pay for transaction authentication and nodes that run the GAS, there is a GAS fee.
In its initial state, Ethereum relied on proof-of-work for consensus. However, the Casper protocol gradually changed Ethereum consensus from PoW into a custom proof of-stake protocol.