A proper Pit Bull Terrier is a dog that started out as a Bulldog back in the days of bull and bear baiting but as dog fighting bacame more popular it was realised that the smaller dogs had more stamina and agression so were bred smaller for that purpose. They used to be called Pit Bull Dogs up untill about the sixties in the US, they aren't Terriers at all but are a breed in their own right that is just not recognised by the Kennel Club despite having centuries of tracable pedegrees. The one I owned could be traced back to the States and then back to dogs from Ireland that crossed the pond during the Irish potato famine.
So the short answer is that they are Bulldogs.