Sounds like an easy question, until you start realizing how many elements are in your book. It took me a while to figure this out for one of my first books. Since I was talking to a friend about it on twitter it brought the issue back to the forefront of my mind. I recall this coming up last year when I went to my first writer's retreat. We all sat down around the table and our teacher (a NY Times Bestseller who seemed very intimidating at the time) asked us to tell the class what genre our book was and talk a little about it.
The only reason I didn't crawl under the table and try to hide was because I was already knee deep in the query process trying to find an agent and kind of had an idea of how to answer the question. So went around the table, everyone described their book and said what the genre was. Some had trouble, they weren't sure, just like myself in the beginning. Our teacher told us to think of where we might find it on the shelf in a bookstore. That really helped clarify it for some. Others still weren't sure so we went into a discussion about the main driving theme of the book. What is the most important thing about your book? Is it the love story? The mystery? The adventure? Or is the main theme a fantasy world, or futuristic? This sealed the deal and suddenly everyone knew what their book was.
Don't worry about all the crazy sub genres you hear about out there. They are enough to leave you realing in confusion, I know they often do so to me. Find the heart of your book and you will have found the genre. I once read an agent's blog who said, if you do not know the heart of your book, how can anyone else?