The only breed recognized as a pitbull by the United Kennel Club is the American Pitbull Terrier. The Staffordshire Terrier is a recognized breed that closely resembles a pitbull and was used for bull baiting just like the pitbull.
The original pitbull breed came about in the 1800s when people mated bulldog breeds and terrier breeds together. The result was what is now called an American Pitbull Terrier. The Staffordshire Terrier is also a combination of bulldog and terrier breeds. Because of the pitbull's aggressive reputation, the American Kennel Club will only recognize the Staffordshire Terrier breed and not the American Pitbull Terrier.