What's a book blurb?

I guess I'm not the only one who has seen many book blurbs without knowing their name.

They're on many books, promoting them (yes, it's marketing!):

A blurb is a short promotional piece accompanying a piece of creative work. It may be written by the author or publisher or quote praise from others.

Source: Wikipedia

If you are interested in book blurbing then you can read this article: