Because the visual system is so complex, a problem in any one of its many components can lead to blindness or other visual problems. I'll start with disorders of the eye and move through to the brain.

Disorders of the Eye

Glaucoma

Glaucoma is a condition of high intraocular pressure resulting from poor draining of aqueous humor. Aqueous humor is a fluid similar to blood plasma and provides oxygen and nutrients to the lens, cornea, and other parts of the eye where opaque blood vessels would interfere with vision. This fluid circulates primarily through the anterior segment (front) of the eye, and if it does not drain properly the pressure in the eye builds up and pushes on the retina and optic nerve. This damage can be very subtle and painless, and irreversible damage may occur without any symptoms. A glaucoma test is standard at most optometrists' offices. It's painless (though a bit unnerving) and involves bouncing a tiny puff of air off the eye and measuring how much the eye deforms.

Myopia

Also known as near-sightedness, myopia makes seeing distant objects difficult. Myopia results from an eye that is too long from front to back, like a football. The lens is unable to compensate for the increased distance and the image ends up focusing too far in front of the retina. Corrective, concave lenses (either eye glasses or contact lenses) can fix the problem.

Hyperopia

Hyperopia (or far-sightedness) results from an eye that is too short or disc-like. It can also occur due to age because the lens is continually growing, and after a long time it can become too thick and inflexible to focus properly on near objects. Corrective "reading glasses" are available, and—as the name implies—generally only need to be worn when looking at something up close, such as a book. If a person suffers from both myopia and hyperopia (typically in old age) they can wear bifocals. The main part of the lens corrects for nearsightedness, while the lower portion corrects for farsightedness (arranged this way because one usually looks forward or up to see distant objects and down to see small, close-up objects).

Astigmatism

An astigmatism is the result of uneven curvature of the lens and/or cornea. That is, the lens may be rounder from top to bottom than it is from left to right (or vice-versa), leading to focal lines instead of focal points. Corrective lenses are ground cylindrically to accomodate this problem.

Cataract

A cataract is a clouding of the lens. Cataracts can be caused by aging, diabetes, or prolonged ultraviolet B radiation (usually from the sun). Smoking can also contribute to cataract formation. It is believed that the clouding of the lens is caused by nutrients failing to reach the deeper parts of the lens. Treatment of cataracts is done by surgically removing the lens and replacing it with an artificial lens.

Detached retina

A detached retina can often result from head trauma or extreme G forces and leads to the retinal layers separating. The vitreous humor can seep between the layers and lead to permanent blindness if not treated. Treatment consists of laser or cryosurgery to reattach the retina. When a person's retina is detaching he or she may perceive small flashes of light from retinal cells firing at random.

Diplopia

The condition of uncoordinated movements of the external eye muscles (those that move the eye) is known as diplopia. It is sometimes refered to as lazy eye or crossed eyes, depending on how it manifests itself. This condition can sometimes be treated with eye exercises to strengthen the weak muscles, or wearing a patch over the good eye to force the weak eye to work harder. Prolonged diplopia can sometimes lead to the brain ignoring the bad eye, causing functional blindness in that eye.

Color blindness (protanopia, protanomaly, deuteranopa, deuteranomaly, tritanopia, tritanomaly, and rod monochromacy)

Color blindness is a blanket term for several color perception problems which are detailed on the color blindness page.

Disorders of the Optic Nerve

If the optic nerve is severed the brain cannot receive visual information from the eye it was connected to. Less severe damage can result in impaired vision to all or part of the visual field. Damage to nerves and the brain are usually permanent and untreatable, although some natural healing and accomodation may occur with time.

If the optic nerves are damaged beyond the optic chiasma then the contralateral (opposite) half of the visual field in both eyes can be affected. That is, if the optic nerve is damaged on the left side the right half of the visual field in both eyes will be impaired.

Brain Damage

Primary visual cortex

Damage to the primary visual cortex (located in the occipital lobe at the very rear of the brain) results in complete or partial blindness. Small areas of damage will only affect portions of the visual field, and the side the damage is on will result in problems with the opposite side of the visual field (e.g. left occipital damage will result in blindness in the right half of both eyes).

If blindness is the result of damage to the eye or optic nerve then it is possible to implant a device into the occipital lobe to electrically stimulate the neurons in this area based on an image from a video camera. The visual accuity is very poor and is only in shades of gray. This treatment is still experimental and not widely used.

Damage to the occipital lobe will result in the person not being able to consciously see things, however there is an interesting effect called "blindsight" where other parts of the brain do seem to be gathering some visual information despite the damage. If a person with blindsight is asked if they see an object in front of them they will say no, however, when asked to pick up the object they find to their surprise that their hand has successfully reached for the object and picked it up, indicating there are other visual pathways that our consciousness is not aware of.

Achromatopsia

Color blindness is not always the result of retinal irregularities. It can also result from damage to the visual association cortex, located in the medial (central) occipital lobe. Not only is color perception affected, but patients are not even able to remember what colors looked like prior to the damage.

Apperceptive visual agnosia

People with apperceptive visual agnosia may have perfect visual accuity, but they cannot perceive objects by their shape. They can still navigate a room or pick up objects, but they cannot tell you what they are. Milder forms of this impairment may result in the inability to name very specific classes of objects. For instance, someone may be able to identify body parts, but not common animals.

Prosopagnosia

A less severe form of apperceptive visual agnosia is prosopagnosia. Sufferers can identify most objects, but they cannot recognize peoples' faces. They know they are looking at a face, but cannot tell you who they are looking at. Sufferers of prosopagnosia rely on secondary clues such as manner of dress or the sound of the person's voice to identify the person.

Associative visual agnosia

A slightly different disorder is associative visual agnosia, which makes naming objects difficult. The person perceives the objects just fine, but cannot tell you what they are. Their knowledge of the words are still intact, but they cannot form an association between what they see and what the name is.

Balint's syndrome

Damage to the parietal-occipital region of the brain (just to the side and forward of the visual cortex) can result in Balint's syndrome which results in optic ataxia, ocular apraxia, and simultanagnosia, which are explained below.

Optic ataxia

Optic ataxia is difficultly in coordinating movements based on the objects one perceives. For example, reaching for something in front of the person is very difficult. The hands may reach in the wrong direction or the fingers will not open to the correct size and orientation to grasp the object.

Ocular apraxia

People with normal vision will typically shift their eyes from one object to another, looking at some things longer than others, and thus taking in the whole environment. Sufferers of ocular apraxia can't guide these motions. The eyes move about somewhat randomly. Fixing on one object is difficult as the eyes keep moving.

Simultanagnosia

This is the inability to perceive more than one object in the visual field at once. Even if two objects are overlapping or right next to one another the person will see one or the other, but not both.

Sources: "Foundations of Physiological Psychology," 4th Ed., by Neil R. Carlson
      "Human Anatomy & Physiology," 4th Ed., by Elaine Marieb.