The Credit was an inter-galactic currency used by many planets. They can be stored on special "Credit Cards'". It is not known how credits are compared with other currencies, howether The Doctor said that £1,000,000 is equal to 50,000,056 Credits in 2007. (DW: Voyage of the Damned)
Credit is used thousands of years on, in the year 200,000. (DW: The Long Game)