Any novel is made up of many different elements. There's the story itself, and the cover of course, and you can't forget about the title or the blurb. Then maybe you'll need a trailer, a whole marketing campaign, an idea for a sequel...it's a lot to do. But while you're doing all that, don't forget another important element: the log line.
The log line, also spelled logline and log-line, is a one-phrase teaser for your book. Even if you don't recognize the terminology, you've definitely seen log lines before.
A good log line is a great hook for readers, a brief taste of your writing and everything the story has in store for them. The log line can bring the entire cover design together, tempting readers to explore further. To come up with a good one, try to sum up your book in a single sentence, without giving away any sort of spoilers. Vague is okay as long as it's really interesting.
Place the log line prominently on the cover, but not obtrusively. It should work as a part of the overall design, and naturally draw the eye. Remember that it's just a teaser, a little taste, but it's also your one good shot at grabbing new readers. Keep it simple and interesting, and you'll nail it.