The general idea is that you can make your own games that other people can play. But the tools and options for the game creationlook limitless, only bound by your imagination or lack of it. You can even use the eyetoy to take snapshots that can then be used a textures in the game. Have a look:
Here is another video with someone talking through the game design procedure
Finally, an interview from Aleks Krotoski's Guardian site with the makers of Little Big Planet.