Since you're posting this in novels, Jane, I'm going to presume that you're talking about a logline or a blurb.
A logline is what is placed in a catalogue the publisher sends to distributors, wholesalers and bookbuyers for stores that describes the book in about ten words. A logline for Harry Potter might be: Boy wizard learns magic while battling sorcerer who killed parents.
A blurb is what you might see on the back cover of a book. Unless you're self-publishing, it's usually the marketing department or your editor who will write this. Very few authors with small or large presses are given this opportunity, so you don't need to worry about that. If you're self-publishing, then the goal with a blurb is to give "teasers" about the book that will invite people to open the cover to find out more.
On the other hand, are you asking about something like a "high concept" to sell the book to a publisher? A high concept (again using Harry Potter) could be something like: "Lord of the Ring's Frodo meets Dune" or something like that. It's intended to give information about the genre and give an editor some concept of other works the book might be like.
Does that help, or just give you more questions?