The “blurb” is a writer’s slang term for what you see on a book’s back cover. Technically, it’s referred to as “back cover matter,” and it’s designed to grab the interest of potential readers.
The whole idea behind the blurb is very simple. What do you do when you’re in a bookstore? Most people browse their favorite sections. For some, that may be the “new releases” section, and for others, it might be the tables that display the latest works from best-selling authors. Whatever their favorite section, people will usually browse until they see a cover that attracts their attention and then look at the title. If the cover and title look interesting, they’ll pick up the book and read the back cover blurb or the blurb on the inside of the cover flap. Then they decide whether or not to buy the book. So the blurb, in effect, serves as a short sales pitch, and it is a very important part of the overall writing and publishing process. Here’s the blurb for Just by Chance:
When high-class escort Kim Carter goes out for a night on the town with New York mobster Vincent Molini, the last thing she expects to see is his picture plastered all over the news. Molini has been murdered, and Kim was the last person he was seen with the night before. With the help of Kim’s friend Nikki—a talented computer hacker—Kim manages to skirt the police and go into hiding in the Florida Keys. But she isn’t completely under the radar . . . New York crime boss, Frank Famulari, sends a team of his cohorts to find Kim and learn the truth about Molini’s death. 
What I found interesting is that the back blurb is one of the first things a potential reader sees, but it’s often the very last thing a writer writes, which was the case with me. I worked through at least three or four versions of the blurb for Just by Chance until I settled on the one to use.