Ethereum is a decentralized application (dapp) platform and cryptocurrency that allows developers to create their own applications without needing their own blockchain. Created by Vitalik Buterin in 2013, it uses a proof-of-work consensus mechanism but is transitioning to proof-of-stake to increase transaction throughput. Ethereum's smart contracts enable self-executing agreements and drive the development of dapps, which could revolutionize various sectors by providing a secure and immutable platform.