The question of how a blurb differs from a synopsis comes up quite a bit and I’m sure I’ve blogged on it before, but if I’ve learned anything, it never hurts to repeat good information.
Think of the blurb (used in the query letter) as the back cover copy for your book. In fact, if you write a really good blurb it could easily become the back cover copy the publisher actually uses. The blurb will focus on the highlights and, most importantly, the hook or the selling points of your book. It shows the reader what makes your book stand out from all others and excites them into wanting to read more. Only a few primary characters and plot points will be given attention in the blurb.
A synopsis (usually requested with more materials) is a much more detailed description of the book. If you’re writing a mystery, a synopsis will highlight the process of solving the mystery, if you’re writing romance it will focus on the details and foibles of the romance. In other words, a synopsis will show the details of the story, the character’s growth, and always give the outcome.
A synopsis will run anywhere from two to ten pages while a blurb should never be more than three paragraphs, but probably two.