Yes your synopsis may include an intro section that explains the book in general.

The intro should be no longer than 2-3 paragraphs, followed by the actual synopsis.

Keep your synopsis to fewer than 4-5 pages, regardless of your novel's length or complexity.

Just tell the story in brief, summarizing the major characters and plot points.

And yes, you do reveal the "surprise ending" (if any) in a synopsis. The purpose of a synopsis is not to tease the reader, but instead to clearly outline the book itself.