What Makes a YA Book?

I just finished reading a fantastic book called Saving Francesca by Melina Marchetta. I loved this book, and it got me thinking about what makes a book at YA book. Is it that the character is young? That can’t be, because there are lots of examples of books with young protagonists. Is it that the themes are related to young people? But the themes in this book are dealing with loss, grief, love, finding oneself… all the things any great book is about. So what is it then that makes this a YA book? Or is that category not real? Maybe there are no barriers between what youth and adults read. After all, I’m not a young woman, and I read books lablled YA.