In the late 19th century to early 20th century, two clubs were formed for the specific purpose of registering APBTs: the United Kennel Club and the American Dog Breeders Association. The United Kennel Club was founded in 1898, and was the first registry to recognize the breed, with the owner assigning the first number to his own APBT.
The dog was bred first to bait bulls and bears. When baiting bulls was deemed inhumane, ratting (a sport where a number of rats were placed in a pit for a specified time with the dog) and dog fighting became more popular. The APBT was used in both sports, and its prevalence in being put in pits with rats, or other dogs led to "pit" being added to its name.
In America, farmers and ranchers used their APBTs for protection, as catch dogs for semi-wild cattle and hogs, to hunt, and to drive livestock.The dog was used during World War I and World War II as a way of delivering messages on the battlefield.
Though of the same family, the American Pit Bull Terrier diverges in appearance from the Staffordshire Bull Terrier, having fewer bulldog traits in the face and body. The American Pit Bull is medium sized, having a short coat and smooth well-defined muscle structure, but should never appear bulky or muscle-bound. Its eyes are round to almond shaped, and its ears are small to medium in length and can be natural or cropped. The tail is slightly thick and tapers to a point. The coat is glossy, smooth, short, and slightly stiff and can be any color.The breed ranges from a height of about 17 to 22 in (43 to 56 cm) at shoulders, females weigh between 30 and 50 lb (14 and 23 kg) and males weigh between 30 and 60 lb (14 and 27 kg)
Data refer : http://en.wikipedia.org/wiki/American_Pit_Bull_Terrier