The cornea is the clear tissue on the front of the eye and is responsible for two thirds of the focusing power of the eye. (The lens, where cataracts develop, does the other third of the focusing). The cornea and surrounding ocular surface can be the sites of numerous diseases ranging from dry eye to those that require transplantation of the cornea. Because the cornea does most of the focusing of light, changes in its shape or clarity can significantly affect the degree to which the light is clearly focused.
Corneal diseases generally fall into three categories. Those that change the shape of the cornea, those that create scars/opacities and those that are painful. (It is quite common to have all three combined.)
Injuries and infections are the most common causes of scars and irregular reshaping. The most common corneal infections are herpes simplex and those caused by contact lenses. The most common non-traumatic/non-infectious shape problem is keratoconus. This is a familial, progressive thinning and warping of the cornea which initially requires rigid contact lenses and ultimately a corneal transplant in many cases. The most common non-infectious/non-traumatic opacity is Fuchs' dystrophy. This is also an inherited disease and causes progressive thickening and haziness. Salt drops are of some limited benefit and this also leads to corneal transplantation in many cases. Common diseases that can be purely characterized by pain/discomfort are dry eye, corneal abrasions (scratching the superficial surface layer), recurrent erosions (where the surface comes partially off without trauma) and trauma. Safety glasses protect eyes from serious corneal injury.
Laser vision correction involves gently reshaping the cornea so that light comes to a focus on the retina. This can significantly reduce a persons dependency on glasses or contacts.