As I mentioned in a previous post "I wrote a novel. Now what?" genre classification is a touchy subject. There are so many sub-genres out there that it is difficult to decide how to categorize a novel. This matters both when submitting a manuscript to an agent or publisher, and when putting a new book up for sale. I think most stories contain elements of several different genres, and which genre the book gets classified as is pretty subjective.
In my frustration, I've come up with my own method for classifying genre. I call it the vomit scale. It's really quite simple.
A lot of action + little to no vomiting = Suspense/Thriller
A lot of action + a lot of vomiting = Horror
Vomit that glows green, sprouts tentacles, and can power a spaceship = Science Fiction
Vomit that has magical properties that can bring unicorn figurines to life = Fantasy
Characters bury their vomit in the dirt = Western
Characters vomit on stolen jewels in order to uncover secret codes = Spy Novel
Characters do their best problem solving while vomiting = Mystery
Characters are not vomiting at all, but readers have the strong urge to vomit = Romance
And here is how you tell the difference between young adult fiction and adult fiction:
Young adult: the vomiting is described with extremely vague wording and the readers aren't quite sure if the character has actually thrown up or not.
Adult: The vomiting is right out in the open in glorious detail along with everything else.
So there it is, folks. I hope that helps.