What happens to leftover Kickstarter money?

So, I'm curious, what exactly happens to the leftover money from a Kickstarter campaign? Do the creators get to keep it all, or do they have to return some of it to the backers? And if they do keep it, what are the guidelines usual or practices surrounding how they can use it? I'm sure there are a lot of details to consider here, so I'm eager to learn more about how the process works.