Prosopagnosia, or Face Blindness, is a neurological condition that renders a person incapable of recognizing faces. It is unrelated to the person's ability to see faces. Someone with perfect vision can suffer from prosopagnosia. It is also unrelated to the person's IQ.
In the normal brain there is a center that is dedicated to face recognition. Prosopagnosia arises when that special center becomes damaged or is otherwise unable to perform its function.