A synopsis is a summary of your story intended to be read by industry professionals.
This makes it a different text from a blurb, which is designed to be read by the public.
In both cases, you probably want the reader to purchase your story. But the reader of the blurb is merely buying a book or a movie ticket. The reader of the synopsis is taking a much greater risk if they decide to invest in your story.
An editor or publisher or a movie producer or director is accustomed to hearing story pitches. They want to find out as quickly as possible if your story is something that they might be interest in. So they need certain questions answered fast. These questions usually concern the premise (see below).
Sometimes criteria that have nothing to do with the quality of the story per se will cause red flags to wave – for instance a movie producer might stop reading a synopsis as soon as it becomes clear the story is set on another planet or in the distant past. This is not because they don’t like the pitch, but because science fiction or historical movies are simply too expensive for them to produce. An editor will know that certain genres do not fit into the program of the publishing house they represent.
On the other hand, industry professionals are human too. They want to be excited and aroused by a story idea just as the potential audience does. So a synopsis need not be dry and stiff. It can contain similar tricks to arouse curiosity as a blurb.
So as with the blurb, you probably want to include the three elements of a story premise (protagonist, setting, and problem) and may try writing the first draft of the synopsis in the “what if …”, “imagine …” or “in a world …” manner. (See our post on how to write a blurb).
The main difference between synopsis and blurb is that in a synopsis you would probably include how the story ends. The synopsis is not a teaser, but a summary that may contain spoilers.
The synopsis is an essential part of a story proposal (sometimes called exposé), but it is important to remember that a proposal requires much more information – about the author, the audience, the genre, etc.
Related function in the Beemgee story development tool: