1. Visual Illusion is caused by the visual system and characterized by a visual percept that appears
to be different from reality.
2. According to Richard Gregory a classification proposed by him is useful as an orientation that,
there are three main classes: physical, physiological, and cognitive illusions, and in each class
there are four kinds: Ambiguities, distortions, paradoxes, and fictions.
3. An example for a physical distortion would be the apparent bending of a stick half immerged in
water; an example for a physiological paradox is the motion aftereffect (where, despite
movement, position remains unchanged). 
4. An example for a physiological fiction is an afterimage
5. Three cognitive distortions are the  Ponzo , Poggendorff, and Müller-Lyer illusion.
6. Physical illusions are caused by the physical environment, e.g. by the optical properties of
water. Physiological illusions arise in the eye or the visual pathway.
7. The most widely know illusion is visual illusions which are the result of unconscious inferences.