Definitions from my Encyclopedia of the Dog:
American Staffordshire Terrier
Breed History: Originally identical to the British Staffordshire Bull Terrier, the American was selectively bred for greater height and weight, and bulkier build. In 1936 it was recognised as a seperate breed.
"Like it's close British relative, the American Staffordshire Terrier can be extremely gentle and affectionate with children and adults, and at the same time potentially lethal with other dogs.
The breed has unusual strength for it's size, and all Amstaffs need early socialisation with other animals to ensure they do not follow their instinct to attack. The breed is almost always a loyal and obedient canine member of the family. It does, however, descend from bull biters and pit fighters, and still has the jaw power and tenacity to inflict horrible wounds."
American Pit Bull Terrier
Breed History: Throughout the world dogs have, at one time or another, been bred to fight. This breed descends from the Staffordshire Bull Terrier crossed with other fighting dogs, including the extinct fighting Bulldog.
"Hysteria is now associated with this breeds name. Banned in Sweden, neutered, muzzled and microchip identified in Great Britain and prohibited from entering some areas of North America, the Pit Bull is as much a victim of the media as it is of it's historic breeding to fight other dogs. Some unscrupulous owners, taking advantage of its natural tenaciousness and strength, continue to use abusive training techniques to produce fighting Pit Bulls, willing to take on any animal and even humans. When trained from an early age to obey commands, the majority of the breed prove to be companionable even gregariously fawning dogs."