I got this question recently:
“What is the difference between a screenplay treatment and screenplay synopsis?”
Typically a treatment is a much longer and more detailed description of the screenplay than a synopsis.
Treatments are often written before the script is written as a guide for the screenwriter and producer so that the screenwriter doesn’t waste time writing a draft that the producer isn’t going to like. In fact many paid writing assignments have a payment schedule which includes the treatment as a step towards the first draft. I’m a big proponent of outlining your story before turning it into a screenplay, but there really isn’t a good reason to write up a formal treatment if you’re writing your screenplay on spec.
Typically a synopsis will be a less than one page summation of your screenplay’s story. As of right now, I recommend only including a log line in your query letter, not a full 1 page (or shorter) synopsis. What I’ve found is that if the log line is good that’s enough to get a script request, and giving them more than that just gives them more reasons to say “no.”
In some cases a producer or reader will ask for a short synopsis of your screenplay to read before they agree to read the entire script. So it’s good to have one written. I also include synopses of all my scripts on my screenwriter website so if a producer is checking out my site they can get a good feel for which projects might be right for them. I also feel like a short synopsis really helps you boil your story down and decide what’s important, so it can be a very helpful tool in trying to do re-writes, too. Also, in the event that you option or sell your screenplay, almost certainly the producer will want a short synopsis to help him raise financing. And if you have a well written synopsis ready to go, that can prove helpful.
There really is no exact definition or specific requirements (that I’ve ever heard) that defines what a treatment is or what a synopsis is, but that’s the gist of it.
Check out my post Writing a synopsis for your screenplay if you have any questions about how to write your synopsis.