What makes a book YA?

My dad and I had this discussion last week. He said that he'd asked the library about the categorization of a book that he'd picked up. They told him that they categorized as YA when the main character was a young person. I can understand that, and I agree with it. However, the particular book that was categorized as YA had an adult as the main character. It did have some violence, although not gratuitous. There wasn't any sex or questionable content either.

Maybe there have been other discussions about this. I haven't come across any as of yet. If you know of any, please send me a link. Otherwise, I'd love to hear your thoughts. What makes a book young adult as opposed to children's or adult?