Basically, you pledge to preorder or donate money to create a product, movie, album, or service. If the service gets enough pledges, it is charged. Really cool idea for indie-published music, documentaries, games, products, and other things you're trying to launch on a budget!
I like it There's a few sites doing similar stuff (donations to pre-fund an album/movie/whatever) but you pay whether it gets enough pledges or not, so the idea of it only going through if it actually gets enough pledges is really cool.
Edit: Pledgemusic was the main one I was thinking of: http://www.pledgemusic.com/projects/katehavnevik