Here are the basics--a sort of 'how to' guide--and then I'll share an example of what I came up with.
You have one page to get the agent to read your book. That's it.
Paragraph One--The Hook:
You have one sentence, or possibly a couple, to pique interest and draw the reader in. This is not as easy as it sounds. Write and re-write.
Paragraph Two--Tiny Synopsis:
Now you have 150 words, maybe 200, to summarize your entire novel. This one feels like parasites in the brain where the harder you work, the hungrier all those little white worms are going to be and the more cheese holes they're going to bore. It's excruciating. Just keep on working it over. It's an extended hook.
Paragraph Three--Writer's Bio:
Keep the egocentric stuff related to writing and you're probably okay here. Brag a little, but don't be an arrogant douchbag about it. If you have some sort of writing degree mention that. If you have publishing credits, mention those. Writing awards are good too. Don't go overboard, but don't be too modest either.
And then close by thanking the agent for his/her time and consideration. Then mention that the full manuscript is available upon request (it's finished, right? Right???).
Then, you're done. Send it out to a few agencies and await the inevitable rejection letters.
Here's an example of my own fumbling, trying to summarize my novel and make it appetizing:
The current incarnation of my query letter is, I believe, passable, and I tried sending it out to a few agents and agencies and now await responses, but I'm still not entirely happy with it. I think it needs some more work. It's lunch time for those parasites and I need to get cracking while I still can. I can feel them swimming around up there...