Perceptual constancy allows us to perceive an object as roughly the same in spite of changes in the actual image. For example, a ball thrown away from us appears to be the same, even though its image is shrinking in our visual field.
Theories regarding perceptual constancy are really not important to the photographer; rather it is the insight that perceptual constancy can work against the photographer as he composes his shot. Our unconscious assumptions delete information on changing perspective and light, and we need to "decompensate" to see the picture elements as they really are.
What follows is adapted from Encarta.
Lightness constancy means that our perception of an object's lightness or darkness remains constant despite changes in illumination. Thus what appears bright to us might appear as dark as it really is on film.
Color constancy means that we perceive the color of an object as the same despite changes in lighting conditions. You have experienced color constancy if you have ever worn a pair of sunglasses with colored lenses. In spite of the fact that the colored lenses change the color of light reaching your retina, you still perceive white objects as white and red objects as red.
Shape constancy means that you perceive objects as retaining the same shape despite changes in their orientation. The flat front surface of a book looks rectangular even from an angle which renders it 2D shape as a trapezoid..
Size constancy is the tendency to perceive objects as staying the same size despite changes in our distance from them. For example, when you see a person at a great distance from you, you do not perceive that person as very small. Instead, you think that the person is of normal size and far away. Similarly, when we view a skyscraper from far away, its image on our retina is very small-yet we perceive the building as very large.