They want to know on what shelf it will be in Barnes & Noble.
One of the reasons, I suspect, is because if you've written a erotic thriller and they only represent children's picture books, they won't have to waste much time. So, you may think, maybe I should do a broad spectrum of genres, so I might hit on one or two they represent.
Not a good thing. First, if you are serious about querying agents, you need to do your homework and find out if the agent represents your genre. There are plenty of websites that have already done this for you, but when you put the estamp on the email and send it on its e-way, you'd better have double checked the agent's website.
Second, if you say, "Well, it is kind of an action/adventure, sci-fi, multicultural, romantic thriller, western, detective book," the agent may think, "Is there one section at Barnes & Noble for all that?" Why give the agent a reason to reject your query just for that? They get hundreds of queries. Don't give them a reason to discard your hopes before reading a sample of your writing.
You wouldn't go up to the Jack-in-the-Box and say, "I'd like a bacon sandwich, a lettuce sandwich, with a slab of ground beef, and some tomatoes, and I'd like to have some onions, and dill pickles would be good, and I'd also like some cheese with mayo and toss in some mushrooms with catsup," would you? If you did, you'd better hope Jack is open 24 hours, because it will be a while before he knows what you really want.
If you say, "I'd like a cheeseburger, with bacon and mushrooms," Jack immediately knows what you want.
Have you written a book like the ones you like to read? Go to your local bookstore and ask what section those books are in. That's your genre.
Give the agent the same respect you'd give Jack.
There can be drawbacks to that. I went to B&N once when I was thinking of writing a book similar to the type of writing Janet Evanovich does. I asked what section I could find the newest Janet Evanovich book.
The clerk pointed to the front of the store. "On the best-seller table."
Oh yeah, my genre is Best Seller.
Editor's Note: Melanie Sherman is not suggesting you treat an agent like they are a fast-food, drive-up window.