ON THE DEFINITION OF GENRE

The concept of "genre" can be a fairly slippery one, but allow me to propose that genres of fiction can be broken down into four categories:

Which can roughly be explained like this:

(1) If a book is set in the 14th century, its historical fiction. If a book is set on Mars, its science fiction. If a book is set in a magical fairy kingdom, its fantasy. And so forth.

(2) If a book's plot is significantly based around solving a crime or puzzle, then it's a mystery. If a book's plot is significantly based around two people falling in love, then it's a romance. And so forth.

(3) If a book is supposed to make people laugh, it's a comedy. And so forth. (This one is a bit harder to get your thumb on.)

(4) If a book is meant to be read by children, it's a children's book. If it's meant to be read by teens, it's a young adult novel. If it's not meant to be read by people younger than 18ish, then it's a mature novel.

You can freely mix-and-match between the different types of genres (a historical children's mystery comedy). Mix-and-matching with another genre of the same type is bit trickier. I think the only genre which is truly exclusive is setting.