Nodes and clients

Node is simply a computer, which participates in the Ethereum network. It stores the blockchain data, and also may verify a transactions on a blockchain (but doesn’t have to).
The Ethereum network is made up of many nodes, which are connected together.
Each runs compatible client software. There are two clients that are used by a majority of nodes:
Geth (written in Go)
OpenEthereum (written in Rust)