If the project is not funded in time, then it does not happen and the charges for the money are dropped. It is a relatively low-risk way to get funding or support a project. Some projects have had a landslide of backing, others do not. The biggest draw to entice backers is the rewards. The rewards vary from project to project and may be as small as a piece of the completed item to a hand in its creation.
An article from Mashable.com talks about tips for raising money on Kickstarter. These can range from proper planning to proper awards. Since there is a reduced risk, if your project does not turn out to be as popular as you would have liked, you can try out different ideas and see what will work.
You can read the entire article on Mashable.com here.
If you are interested in browsing or joining Kickstarter click here.