• The ancestors of modern Pit Bulls come from the bulldogs and terriers of England. At one time, every county in England had its own breed of terrier.  Terriers served an important purpose in England by killing vermin that might otherwise ruin crops, damage property, or spread disease such as the Black Plague. The development of sports such as rat- or badger-baiting further added to the breeds' importance.
• Another common misconception is that Pit Bulls don't feel pain. Pit Bulls have the same nervous system as any other breed, and they can and do feel pain. Historically, breeders propagated dogs who would tolerate or ignore discomfort and pain allowing them to finish required tasks. This trait is known as “gameness” which is defined as “The desire to continue on and/or complete a task despite pain and discomfort.” Therefore care must be taken to avoid serious injuries, since Pit Bulls, like some herding dogs, will continue to perform tasks despite injuries as severe as broken legs.
• Pit Bulls are often used for dog fights, due to their strength, courage and widespread availability. Fight training often means using other dogs of the same sex, as most dogs will not show aggression towards the opposite sex. The true "pit dogs" have excellent attitudes towards humans and show aggression to other animals and other dogs. Although dog fighting is illegal in the United States, it is still practiced, and is sometimes accompanied by gambling. 

Source: Wikipedia