The American Pit Bull Terrier is a breed of dog, originally bred for the cruel sport of dogfighting. Due to their violent origins, they were bred to have a powerful jaw, capable of delivering up to about 320 pounds of force (third strongest after Rottweilers and German Shepherds).
Although dogfighting is outlawed in most of the civilized world, it still unfortunately goes on illegally, and as some pit bulls are bred for fighting, and often mistreated, they can be aggressive due to their perception of humans as mean and cruel. Constituting only a small portion of the dog population, they account for nearly half of fatal dog attacks in the United States. Despite their disproportionately high number of attacks, they are usually no more aggressive than other dog breeds, and are naturally sweet and gentle and friendly and can make great pets if they are treated well. There is also the fact that they are not a breed, but rather a mix of different breeds. In fact, "Pit Bull Terrier" is just an umbrella term for any dog that is a mix between a bulldog and a terrier, but there are no standards for which breeds or how much of each they have, as they are not officially recognized by any major kennel club. This is why pit bulls vary so much, both in appearance and temperament, with some being nice while others are mean. There is also the fact that while there may be sweet and gentle individuals, they were bred to fight, so they may have a somewhat aggressive nature, and may play fight for fun, which is why they are not always good around children, strangers, or other dogs. Although any dog could potentially be a good pet, these facts must be considered first, and certain precautions must be taken, such as sending them to puppy school as soon as possible.