The credit was a currency used by many planets. Apparently, many different civilisations had different values.
The Stoish civilisation, for example, had credits which were stored on "Credit Cards'". The Doctor said that £1,000,000 was equal to 50,000,056 credits in the early 21st century, meaning that the average credit was roughly equivilent to 2 pence. (DW: Voyage of the Damned)
Credits were also used by the various Human Earth Empires, at least until the year 200,000. At this point a Cronk Burger costed 2 credits 20. (DW: The Long Game) These credits apparently had a different value, as in 4125 the cost of an Ood slave was 50 credits. (DW: Planet of the Ood)