Agnosia is Greek and means "failure to know." Visual agnosia occurs when the visual association cortex is damaged; there are three types of visual agnosia.
Apperceptive visual agnosia is the failure to perceive objects, even though visual acuity is retained at a normal level. Apperceptive patients do have the ability to distinguish between size, brightness, and hue, but they fail tests that require matching or copying an object. Simply put, an apperceptive patient will claim complete blindness, but if you ask him to ignore the fact that he can't see anything and tell you what color the ball in your hand is anyway, he'll get it right every time. What he lacks is conscious vision.
Victims of prosopagnosia suffer from the inability to recognize faces. The patient will know that what they are looking at is a face, but will be absolutely unable to tell you whose face it is, even if they've seen the face hundreds of times. There is disagreement among researchers about whether or not propagnosia is simply a less quantitatively severe case of associative visual agnosia (see below). Because some scientists think that there are very specific areas of the brain that recognize faces, propagnosia is still its own category.
The third type of visual agnosia is associative visual agnosia. People who have associative visual agnosia cannot recognize objects that they are concentrating on. For instance, a farmer with associative agnosia would know what a cow is and what a cow looks like, but if you put a cow in front of him and asked him what it was, he would draw a blank.