the term PITBULL is used to categorize a number of breeds, including the American Pitbull Terrier, the American Staffordshire Terrier, and the Staffordshire Bull Terrier.....it is not just ONE breed though...alot of times people think it means APBT..but it doesn't...it can mean any of the three breeds I mentioned...
The American Pitbull Terrier was in fact the first "pitbull"..it was used for fighting in Brittain, and then transported to the states and renamed AMERICAN...from that, people decided to begin showing the dog and registered with the UKC...renamed the breed, and after years of selective breeding, it became known as the AMERICAN STAFFORDSHIRE TERRIER..in reality, the APBT and the Staff Terrier..are one in the same... the main difference is that one was used for fighting, the other became a show dog..hence differences in appearance and certain characteristics.... they are different breeds of dog, but fall into the same category known as the pitbull...
The staffordshire bull terrier is quite a bit different in appearance pretty much for the same reason..it was bred for a different purpose but again, stems from the bulldogs that the american pitbull terrier was born from...
I hope that helps...there's alot of confusion with this whole pitbull thing..lol