It's a term that generally refers to a couple of different breeds. The American Pit Bull Terrier and the Staffordshire Bull Terrier are the two breeds that are generally referred to as a pitbull. The American Bulldog and the Bull Terrier are frequently lumped in there too.
The problem with pitbull attacks (from what I have read) is that it's usually the victim that ID's the breed type. Most people have no idea what breeds are actually lumped into the pitbull category and also have no idea what a "pitbull" actually looks like. For example:
http://www.pickthepit.com/
