What is the difference between a ghost written autobiography and a biography?

I know there are people out there that just can't write a book, i.e. Paris Hilton, that have "autobiographies" that were written by a ghost writer. I just looked up an article on it, (http://en.wikipedia.org/wiki/Ghostwriter ) and even Ronald Reagan has one out. As I can tell, a ghost-written autobiography might at least have some basic outline by the person actually claiming to be writing it. I understand the whole marketing aspect of it but it really just comes down to can you still call something an autobiography if it wasn't written by the actual person?