The main thing is that cryptocurrencies run on their own blockchains whereas a token represents an asset or utility existing on top of a blockchain. In this case, it's Ethereum blockchain. ERC20 is a token standard, the most widely used one. In 2015, Ethereum issued technical specifications for tokens on the Ethereum blockchain. Tokens that conform to these specifications are known as ERC20 tokens. (ERC stands for Ethereum Request for Comments.)