(Or alternatively, you can go to a conference and pitch an agent in the flesh. Go here for a list of conferences: Writing Conferences )
Before you send your query, you need to do a little research. Does the agent accept your genre? Does the agent have a track record? Does the agent clearly describe his or her submission requirements? And when the agent starts describing what it is he or she wants, what on earth are they talking about?
Agents, like publishers, use jargon - it's one of the hazards of the trade. But as an author, you may have no idea what they mean by "high concept," "upmarket," "literary." Speaking reasonably, your job is to write your book; theirs is to find a niche for it. Unf, writers are also expected to define not only their genre, and their audience, but also their market niche, which is something they may know nothing about.
It's never too late to learn!
Here are some terms you may run across on agent bios when they describe what it is they are looking for.
High Concept means the book can be made into a movie. In general books that fall into this category have a single premise ("what if..."), clear story lines, are highly visual, appeal to a mass audience, and have a well-defined emotional focus that fits into a movie category (Family comedy? Drama? Romcom?). If you can sum up your book in one sentence, you may have written a high-concept novel.
Up-market fiction is any novel that has mass appeal and is also well-written. Memoirs of a Geisha falls into that category. These are books you want to keep. Frequently, non-genre fiction may be used to mean up-market.
Commercial fiction is entertaining, has a plot that moves right along, and may or may not feature writing that makes you cringe. The Stephanie Plum mysteries would fall into that category, as well as most popular mass market paperbacks. (Romances, in particular.) These are books you read in a dentist's office, because you found them there, and which you will probably leave in the waiting room after your teeth have been nicely polished. Most genre fiction is commercial.
Literary fiction is art. Almost every writer who has won a Nobel Prize fits into this category, as well as a few who should have but didn't. (Pretentious wannabes also fall into the category of literary fiction, but people often can't tell the difference.) In literary fiction, the way you tell a story is more important than what actually happens. (What exactly happened in White Noise?) The exploration of character, style, and theme is what moves these books along. If you are reading a book, and you have to stop because the prose is stunning, revelatory, or just plain deep (and you are not stoned) you are reading literary fiction. If you need a half hour to explain what's in your book, you may have written something literary.
Narrative non-fiction is any non-fiction book that reads like a novel. The Poet and the Murderer is a great narrative non-fiction book that tells the story of how ... no, I won't spoil it for you. You'll just have to read it.
Strong platforms are what agents representing general non-fiction like to see (though not necessarily narrative non-fiction). Are you the CEO of a Fortune 500 corporation? Have you been a quarterback in the NFL? Are you a surgeon doing experimental brain transplants at a famous hospital? Does everyone on the planet know who you are? Those are people who have strong platforms. If you have a few thousand followers on Twitter, or a blog with a couple thousand followers, or lots of "friends" on Facebook, you do not have a strong platform - although all those things are important to mention if you write any kind of fiction.
These articles will help you find an agent who is right for you:
How to Research an Agent
Beggars Can Be Choosers - How to Pick an Agent
Valuable Tips for Pitching to an Agent or Editor
Finding an Agent – Look before you leap
Are You Ready to Contact an Agent? Take This Short Quiz and Find Out
What Not to Do When Contacting an Agent