Creampie, or internal cum shot, is a colloquial term in pornography
to describe when a man ejaculates inside his partner's vagina. In pornography
vaginal sex is generally followed by a facial or other visible ejaculation. This, coupled with the fact that the wearing of condoms for vaginal and anal intercourse is now commonplace in pornography
-- whereas a creampie by its very nature cannot be accomplished while wearing a condom -- makes ejaculating in the vagina a novel situation which can be advertised to attract viewers.
An anal creampie (other terms: chocolate creampie, mudpie) is a variation where a man ejaculates in his partner's anus.