Before I start this business of why a synopsis is important if you want to publish your manuscript, let me just say: I have now, officially, seen it all.
I live out in the country (way out in the country) and I've seen everything from hot air balloons in distress landing in our front yard; jet fighters from Fort Bragg using our house as a "target"; a huge barracuda in the middle of our road (we are 50 miles from the coast--you do the math); to dogs and cats abandoned on our road because people think no one lives there so no one will know (personally, I think good old Guido ought to break both legs and arms of people who "drop off their pets" in the middle of the country--then Guido ought to drive THEM out to the country and "drop them off" in the middle of a Kansas corn field).
Anyway, so today I was going out to buy dog food and got to our mailbox a half a mile from the house and what do I see? A cow. Yes, a cow--complete with udders looking suspiciously like she needed milking. So...someone is dropping off cows now? How exactly does one throw a cow out of a car?
Thankfully, by the time I got back from buying dog food, she was trotting on down the road towards who knows where--although I did have a moment there when I was wondering if I could somehow sneak the idea of adopting a pet cow into the conversation with my husband... "Oh, dear, I found this poor, sweet animal by the mailbox, and can I keep her?" I had an ulterior motive, you see, besides the milk aspect. I grow roses and cow poop is a great fertilizer...
Okay, anyway, this has nothing to do with writing, mostly because I could never work this into a story and have anyone actually believe it. I just had to write it down, though, because I'm having a hard time believing it, myself.
What the heck is a synopsis and do I need one?
A synopsis is a brief 1-5 page description of your story, and yes, you need one. A synopsis is not to be confused with the small "blurb" you insert in your query letter. A blurb is like that paragraph on the back of a book which makes you want to read it. Theoretically, the blurb in your query letter makes the editor or agent ask for your manuscript. Then, when you send your manuscript, you send them the manuscript itself and the synopsis.
The synopsis tells the editor or agent how your story plays out, including the ending. You do not under any circumstances end your synopsis with a stupid question such as: Will Jason and Evelyn finally resolve their differences or will they end their days lonely and unable to bear the sight of each other? No, no, no. The point of the synopsis is to show the editor or agent that you can write a story with a beginning, middle, and satisfying ending. Do not end with a question. A question is the equivalent of eating a salad for dinner. It is not satisfying and doesn't demonstrate anything other than that you are extremely irritating and unfulfilling.
So, you whine, how do I squeeze my beautifully complex 500 page story into five measly pages? It's easy. First off, you don't mention anyone other than the hero, heroine, and possibly the villain (if there is one). No one else--and certainly not by name. They are unimportant for the purposes of the synopsis. Because what you will be including in your synopsis is the emotional journey of your hero and heroine. That's all. No subplots, no extraneous details.
I used to write the bare bones story turning points for my synopsis and although it did condense the story down enough to fit into five double-spaced pages, I had a hard time getting agents and editors to ask for more than a partial. I realized something was wrong. I took a bunch of classes and technically, my synopsis was perfect. Then I realized what the issue was--although I was including the turning points in the story, I wasn't showing the emotional journey and that is what--actually the only thing that--is important.
Here is an example. First, I'll give you my original method of stating the initial plot point for the synopsis.
After the death of his brother, Joe flew to New York, determined to prove who had killed his brother, and why.
Okay, that sets the scene, but it's kind of boring. Now, here is that same plot point, showing the beginning of the emotional journey of the hero.
Enraged by the death of his brother and ineptitude of the police, Joe flew to New York, determined to find the killer of his brother and exact his own brand of justice.
You see the difference? Which one makes you want to find out what happens next?
There are a lot of people who say the synopsis is not important, but I can tell you from personal experience, if you don't show the emotional journey, and just cover the major plot points in your manuscript, editors and agents will be less likely to request it. I have discovered this through years of trial-and-error. It simply isn't true that a new author can afford to have a technically perfect, but "blah" synopsis. Or worse yet, a rambling hard-to-follow synopsis which ends in a question instead of showing how the story plays out.
The secret is to show the emotions of your characters and the major turning points. Forget the subplots, even if they are clever and wonderful. There is no room for them in the synopsis. What you need to do is break down your manuscript. In a 400 page story, there are most likely three turning points. Those are the core items in your synopsis. In addition, if you are writing a romance, you must include: the moment the hero and heroine meet; the first kiss or step toward intimacy; the first love scene or other moment of "closeness"; the black moment when all seems lost; and the conclusion/reconciliation.
Write down those points in an outline and then frame them with: the hero/heroine did x which resulted in y. This made him/her feel z and s/he decided to do m.
That is, you are writing about actions the hero/heroine took which resulted in a certain outcome. Then you describe how the hero/heroine felt about that outcome and what decision they made because of it. From that decision, another action is borne, carrying them on to the next plot point.
I can tell you from personal experience that if you need to write a short (1-5) page synopsis, which is what most editors and agents want now, you can't just write it by the seat of your pants because you will end up with a very long synopsis. You need to take a sheet of paper and just write the major turning points/relationship points I've indicated above. Then, flesh them out with the action/outcome/reaction formula. This will enable you to "hit the high points" and hopefully show the emotional arc of your story, without taking twenty or thirty pages to do it.
Remember though, you can't just do a dry recitation of facts: He saw Mr. Green murdered and went to the police. You only have a little room, but you have to squeeze in what is driving your characters to do what they do. It's the emotional journey that will grab the attention of the editor or agent, and that is the point of the synopsis.
Oh, one final point. I've been known to, ah, fudge on my synopsis when the plot really is just too complex for it to really make sense. What you write must make logical sense and you do not want to make the editor or agent ask questions. If this means you have to take some minor liberties with your plot to simplify it so that the conclusion flows naturally, then do so. No one is going to sit down with your synopsis and compare it to your manuscript. They are looking for what is driving your characters and the resolution. They could care less if you say the hero kills the villain when in fact the villain falls out of a twenty-story window.
Anyway, don't panic. It's not as hard as it sounds. It's harder.