Writing a Synopsis

It seems like all writers hate to write a synopsis for their novel, but most agents and editors want a synopsis.

Agent Nathan Bransford gives some advice on writing a synopsis:
How to Write a Synopsis

Agent Jessica Faust at Bookends talks about synopsis and what they are actually used for:
Synopsis
How I View a Synopsis

A synopsis is written in present tense and in third person. It should cover what happens in the story in the same order in which the events occur in your book. (Some people start a synopsis with a paragraph summary of the two or three major characters and what their motivations are, and then summarize what happens in the story. This is also fine.) You should include the major plot twists and must reveal the ending.

Short synopsis are generally up to three double-spaced pages. It is generally fine to single-space if the synopsis is only one page long. Stick to describing the major characters and events, and try to describe how one event naturally flows into the next. As in, Heroine does this, but Villain takes it as a personal insult and retaliates with that.

For a longer synopsis, you can summarize each chapter in a paragraph and include more of the minor characters.

The way I did my synopsis was to work on describing my novel in one sentence. It took several weeks, but I finally realized what the heart of the conflict in the story was and I put it in this one sentence. Once I understood what conflicting wants drove the story, I then knew what was important to put into my query letter. After that, I wrote a one page synopsis, then expanded to a three page (double-spaced) version.