Ethereum is a decentralized, open-source blockchain and smart contract platform.
Released in 2014 by Vitalik Buterin, Ethereum has established itself as a global platform for building decentralized finance (DeFi) applications. Ether (aka ETH) is the native currency of Ethereum, and is used as "gas" to pay for all network transactions. As of today, the majority of DeFi applications are built on Ethereum. Many of these applications have their own subset of tokens, which use the ERC-20 standard.
To learn more about the platform, visit: https://ethereum.org/en/