A programmable network
Ethereum is a blockchain designed to run programs called smart contracts. This makes it possible to build applications that interact with assets and data recorded on the network.
ETH and Ethereum are not exactly the same
Ethereum is the network; Ether or ETH is its native asset. Among other uses, ETH is used to pay fees associated with using the network.
Smart contracts
A smart contract is code that executes according to predefined rules. It can manage tokens, exchanges, digital identities and many other functions.
Ecosystem
Ethereum includes standards such as ERC-20 and ERC-721. These standards help applications and wallets interpret certain assets consistently.