An ERC20 token is an asset which follows the ERC20 (Ethereum Request for Comments) protocol. These assets sit on top of the Ethereum network and are distributed and transacted on the Ethereum Blockchain. Some examples of popular ERC20 tokens are Augur (REP), OmiseGo (OMG) and Brave’s Basic Attention Coin (BAT). To learn more about the protocol, check https://theethereum.wiki/w/index.php/ERC20_Token_Standard