I was recently writing about looking at genre as a collection of clubs you can use for marketing your book. If you play by the rules, you get access to the members of that club and can encourage them to read your work.
The tricky bit is finding which club you belong to. Now, there are tons of genres out there, so I'm not about to go and try to define them all. I will however, link to a collection of genre descriptions.
http://en.wikipedia.org/wiki/Category:Fantasy_genres Because Wikipedia has an article on everything!
https://www.worldswithoutend.com/resources_sub-genres.asp Good long list with Sci Fi and Horror sub genres as well.
http://www.bestfantasybooks.com/fantasy-genre.php Tons of good info here.
Okay, Keryl, I went, I read, I found that I wrote a Romantic Fantasy in an Arcanepunk world. What can I do with that? It's not like that's on the category list at Amazon or Barnes and Noble.
True. Most booksellers aren't quite that precise in their genre categories.
What you can do with this is find communities that like what you write. You can use this to home in on blogs that write about what you write (or better yet, review it!). You can search for groups on Goodreads, Shelfari, Librarything, etc that focus specifically on your sort of work.
In a nutshell: once you know which club you can enter, you can go find the members.
When you are marketing something as specific as a book, you do not want to cast a wide net. Spamming everyone on Earth about your book is not only a waste of time, it annoys readers. (And you don't want annoyed readers.) You want happy readers who are already primed to like what you wrote. Finding them, and dangling your book in front of them maximizes the likelihood of not just a sale, but a good review as well.
So, hope that was helpful!