What happens if a Kickstarter isn't fully funded?

I'm curious, what exactly happens when a Kickstarter campaign doesn't reach its funding goal? Does the money that's already been pledged go back to the backers? Or does the project still go ahead with less funding than anticipated? And what are the implications for the creator and the backers in such a scenario? It's important to understand the potential outcomes before committing to a crowdfunding effort.