The answer to that is both easy and utterly complicated. Your story is what makes the synopsis good. But how do you decide what parts of your story to spotlight in the synopsis? That is the tough part and unfortunately it could be the difference between someone asking to read your manuscript or passing altogether on it.
Now that I'm nearly finished with the editing process I'm faced with having to write a synopsis for my new book. Even if my agent doesn't require a synopsis before he reads it, if he decides to try and sell it I know he'll want one to send to editors. It is inevitable that I will have to write one so I might as well just do it. Sorry to break the news to those of you who were hoping that once you land an agent you'll never have to write another synopsis!
Here's a few things you should ask yourself when writing a synopsis: Why are people going to want to read this book? What makes this book different from others of its kind? Be sure to include the inciting incident (that which sets the book in motion), the major plot points, the character arcs, and the climax/resolution.
I won't really get into the length a synopsis should be, that depends entirely on your agent (or the agent you're submitting to) and their preference. Make sure you know what their preference is. I've covered this in prior posts but it warrants mentioning again. I've seen some agents that asked for a one page synopsis and others that allowed three to five pages. After you've done your research on the agent you're submitting to and thoroughly checked their website and guidelines, if you're still in doubt, ask them.
My synopsis is written and sent off to my Scribe Sisters for critiquing, how about you?