Ethereum is a platform developed by Vitalik Buterin. The platform also contains a crypto asset called Ether (ETH). The platform and the crypto asset are both commonly referred to as Ethereum.
Just like Bitcoin, Ethereum uses mining and issues currency through Proof of Work (PoW). However, there is a possibility of a transition to Proof of Stake (PoS) in the future where the difficulty of mining is adjusted based on the amount and time of currency held by miners (as of September 10, 2019).