The contest is a celebration of the ingenuity and creativity of the worlds premier visual illusion research community. Visual illusions are those perceptual experiences that do not match the physical reality. Our perception of the outside world is generated indirectly by brain mechanisms, and so all visual perception is illusory to some extent.
The study of visual illusions is therefore of critical importance to the understanding of the basic mechanisms of sensory perception, as well as to cure many diseases of the visual system.
Take a look at the 2009 finalists.