Corrective lenses address two kinds of vision problems. One is caused by a misshapen cornea. The cornea provides most of the eye's focusing power, so defects here show up as nearsightedness or farsightednes. The other is typically caused by a hardening or other deterioration of the lens in the eyes. This is called presbyopia and causes the visual deterioration found in middle-aged people who need bifocals.
Most nearsightedness stems from a result of a flaw in the shape of the cornea.
Essentially, the curvature of the cornea is too great for the depth of the eye, causing light to focus in front of the retina. Glasses and contacts correct this flaw by spreading out the light before it reaches the eye, then the eye can pull the image into focus in the right spot. Radial keratotomy is a treatment for nearsightedness that involves cutting part way through the cornea in radii around the pupil. The surgeon intrudes as little as possible into the visual field, reducing the chance that scarring will blur your vision. The incisions cause the cornea to flatten, and the degree of correction can be controlled by varying the depth and location of the cuts.