A while back Alex wrote a post about writing a synopsis. I posted a couple of comments on it, and said some things that turned out to be not such good advice.
I said that you should leave stuff out so that the agent will want more.
I said that you shouldn't reveal all the why's.
I was wrong.
At Thrillerfest I got a chance to talk with Robert Astle of Robert Astle and Associates Literary Management for a quite a while and pick his brain on what it means to write a synopsis.
First of all a synopsis needs to tell all the major plot points and any important subplots. It should describe each of the important characters and important minor ones.
It should give the major plot twists, and important minor ones.
Sounds like it includes the whole book doesn't it?
Not so fast.
If you've ever read the back cover copy inside a hard cover book, or the book description on Amazon.com, then you've seen part of the synopsis. The basis for most of that copy is the author's book synopsis.
If you follow the same style as one of those descriptions, but include the important twists, and the ending, you've got it.
The only thing that you probably want to add, is a hook at the beginning of the synopsis to garner the agent's interest.
I just finished writing a synopsis for both of the books that I am peddling and I think I understand much better what's involved. The goal is to describe the gist of the story in one or two pages so that an agent can get a quick idea if the story is right for them.
If they like the synopsis, they will look at your sample writing, and if they like that, you're probably on your way.
So read a few of those descriptions, and practice your synopsis writing. You're going to need it at some point.