A token is a digital asset that lives on a cryptocurrency. The term token is often used in the meaning of user issued token, in opposition to native token that comes into existence along with the cryptocurrency itself.
See also: Cryptocurrencies supported by Trezor
All cryptocurrencies have at least one token. This is a native token which is created along with the cryptocurrency, is essential to its function and is often synonymous to it. For example Bitcoin has bitcoin (BTC) as its native token, while Ethereum has ether (ETH).
Some cryptocurrencies also support user asset issuance. Such assets are reffered to as a user tokens. For example Ethereum has growing number of ERC20 tokens issued by individual users and companies during ICOs or crowdsales.
It is quite common, that native tokens are reffered to as cryptocurrencies (or less formally as coins), while user tokens are called just tokens.