Breed Specific Legislation - or 'BSL' - is the practice of using laws to regulate and restrict dog ownership based solely on the physical appearance of someone's dog. It stems from the belief that some dog breeds are simply "born bad," and identifying those examples can be as easy as noting certain physical features. In areas where "pit bulls" are restricted, authorities seek out dogs with large heads, muscular bodies, short fur, etc - regardless of their genetic make-up. Throughout recent history, select breeds have been targeted this way and large groups of innocent animals have lost their lives, typically after a well publicized bite incident stirs fear in a community. This reactionary approach to public safety is regarded by experts as a knee-jerk and highly ineffective treatment for tackling the multi-faceted nature of any community's dog issues, but it continues to be the fall-back solution in select countries as well as towns and counties dotted around the U.S.  

It's tempting to compare the practice of BSL with early 19th century Phrenologists, who once believed that a man's character could be predicted by mapping out the shape of his skull. It seems ludicrious now, but the practice of studying external features to determine personality and to predict behavior was widely accepted for several decades until advancements in modern medicine and neuroscience discredited it entirely. We haven't been able to get away from this type of thinking in our culture unfortunately, even though science now understands that it is impossible to predict behavior based solely on appearance.

See Pit Bull Rescue Central's Link for more information