A plot synopsis is a summary of the overall plot of a novel or story. A plot synopsis is usually written after a story is finished, and it is often used when submitting a manuscript to a publisher or agent.
A plot outline is often confused with a plot synopsis, though they are not the same thing. A plot outline is written before a story or book is written, and it helps guide the writing of the story. Because a story may change from the outline after the writing is complete, a synopsis is written after the story is done.