THE ORIGIN OF THE PITBULL (NOT THE RAPPER!!!)

Just look at that MASSIVE head and those mean eyes.
Source: Vitamins For Pitbulls

Pitbulls were created sometime in the nineteenth century when some geniuses in Europe decided to cross a bulldog and a terrier in order to create an animal that combined the strength of the bulldog, with the agility and gameness of the terrier. Visit United Kennel Club and Wikipedia for more on this powerful breed of dog. However, if you are the type who does not like to do research - most people seem to fall into that category these days - just know that these dogs are great fighters and contrary to popular belief, do not come equipped with locking jaws (they just have very an unbelievably strong grip and usually refuse to let go once they clamp their jaws onto something). Breaking an ammonia ampoule and holding it to the dog's nose can cause it to release your leg, arm, or any other part of your anatomy it erstwhile had between its jaws, and go for another part, or someone else's. 
The official line is that they are not more prone than other breeds to attacking humans, but videos like the one below by Massioner Tube will definitely cause you to give them the widest of berths when next you see them on your street.