Like many of you pointed out, realizing what books your story is similar to will help you target a specific house or agent.
One of the most important things to know is what genre your books falls under. In other words, at the bookstore, what shelf in what section would your story sit on.
There's tons of genres for fiction.
The list could go on. Not only that, but you can mix and match genres, though it's vital to know the main category of your story.
For instance, I am writing inspirational (Christian) contemporary romances targeted to Love Inspired. However, I've thought of writing an inspirational urban fantasy. That would be mixing the genres up and might be kind of fun, though a difficult sell.
Debra, in the comments yesterday, mentioned a gothic inspirational. How fun! A totally new genre comprised of some main genres.
What genre do you write in? Ever thought of trying something new?