A token system is the system in which a currency operates in Ethereum. It is a database with one operation which is to subtract X units from A and give X units to B, with the provision that A had at least X units before the transaction and that the transaction is approved by A. Token systems are a very literal implementation of the basic banking system's transaction functions on Ethereum. What separates Ethereum token systems from other blockchains is their ability to pay transaction fees directly in that currency. This is done by Ethereum constantly holding and refunding ether used to pay fees to the sender, refilling its balance by collecting the currency units that it takes in as fees and reselling them.