The job of the synopsis is to tell the story briefly and coherently; to outline the ups and downs of your story, and to introduce your characters and to demonstrate their appeal.
Your synopsis has to be good, because it has a lot of work to do: you’re going to use it to sell your book to an agent; your agent will then use it to sell your book to an editor; and your editor will then use it to sell the book to their publishers and sales people.
When I write fiction I don’t outline in advance, and instead use Word's endnote feature to attach a brief endnote to each and every scene as I write it. When I review my work, a quick read through those endnotes reveals most plot-holes and continuity problems which are then relatively easy to fix; and it provides me with a lump of text to base my synopsis on. But I still find synopses incredibly difficult to write, and struggle every time.
Thank goodness, then, for crime-writer Beth Anderson, who provides some very useful information about synopsis-writing on her website. Countless people have been saved from synopsis-writing hell by her great advice—including me.