A synopsis, also called an abstract, is a short summary of the article or book in question. It should cover the primary points and conclusion.
The primary purpose of a synopsis is to enable the reader to decide whether they want to read the entire work. A synopsis may, therefore, also include a short critical comment, like "this is badly written," or "there are no references to back up the statements made."
The best way to get a feel for synopses is to go to an article index, like ProQuest, and read through some of the abstracts.