Ethereum is a decentralized platform that allows developers to build and deploy various decentralized applications (dApps) using smart contract technology. It was created by Vitalik Buterin in 2015 as an extension of Bitcoin's blockchain technology, with the goal of making it more versatile and suitable for a variety of use cases beyond just financial transactions. Ethereum uses its own cryptocurrency called "Ether" which can be used to pay for transaction fees and services on the network. The platform has become popular among developers due to its open-source nature, allowing anyone to contribute code or ideas to improve it.