Credits were the currency in the future, apparently replacing paper money and coins. Credits were not just in the future, but in the present at different parts of the galaxy.
Transactions occurred by means of plastic cards, which were swiped through a card reader. Card readers, in turn, were located virtually everywhere and also seemed to be available for individuals to have their own. The cards themselves were not bound to one person, and were traded and replaced regularly. Kobra once intended to poison the entire population of Gotham City by sabotaging the production of these credits, but the scheme was foiled by Batman with help from the Stalker.
Zeta was also equipped with a built-in credit card that could generate an unlimited amount of currency, enabling him to move about unfettered.
In the Hub, if a credit had no value, then it was deemed the user was attempting to defraud the business, which was evidence prima facie of fraud and vagrancy.