After commenting on this question, I started wondering what the difference between a plot and a synopsis is, or if they are exactly the same. I'm mainly looking for common usage, but anything else (such as dictionation) is also welcome.
First, a bit of "dictionation":
• The relevant sense of plot is "The general course of a story including significant events that determine its course or significant patterns of events."
• The relevant sense of synopsis is "A brief summary of the major points of a written work, either as prose or as a table; an abridgment or condensation of a work."
For a fictional story, plot and synopsis may coincide; for non-fiction, the term plot ordinarily is not used. In literary criticism, plot of a story refers to a plan of events by which the author makes the story progress. Of course authors of non-fiction also choose what to present, but the result is called an outline, plan, or structure rather than a plot. Thus, in summarizing non-fiction, one uses the term synopsis rather than plot, but in summarizing fiction, either term may apply. Regarding which term to use for fiction summaries, comments below refer to length, or depth, of the summary. I'd expect a synopsis to contain more discussion of literary techniques (such as characterization) than would a plot summary.
A "synopsis" is something written down. It's a summary of what happens in the story; so kind of a shorter version of the story. So, for a decent sized novel, you might write a one-page or half-page summary of what happens. This summary is called a "synopsis".
The "plot" is a far more abstract concept. It's not normally written down as a separate entity from the story. Basically, it's what happens in the story - if you're talking about a story, you might describe its plot, its characters, its character development, its level of descriptiveness, how suspenseful or exciting it is, and so on. In other words, the plot is just one aspect of the story; but unlike a synopsis, it doesn't have an existence of its own, outside of the story.
However, to confuse the issue, in a review of a book or a movie, it's common practice to write "Plot:" as a heading, and follow it with a synopsis.
A synopsis basically sums up what a book or play is about, but the plot is the thing that drives a play.
Synopsis: William Shakespeare's Romeo and Juliet is about two teenagers who fall in love despite being from two families that hate each other and how it leads to their doom.
Plot: William Shakespeare's Romeo and Juliet is a beloved play that is 400 years old, and is the most popular of this man's works. The play opens with a throwback to the ancient Greek choruses, where the actor(s) bring the audience into the action of the play by telling it everything that has come before the action begins, a form of exposition. The first scene is a fight in the street between to two factions, the Capulets and the Montagues, and here is its we are introduced to Romeo, of late unhappy that his romance with Rosaline has gone sour. Here he meets with his friends, Benvolio (a kinsman) and Mercutio (best friend, and cousin to the Count)
The simplest way to distinguish between the two, especially if you are pressed for time (such as on test) is to look at the level of detail. A plot will show in detail what is happening within the play, and often in print it will discuss what is happening in the narrative, but a synopsis will condense the most important points into a paragraph or two.