A cataract is when the lens of the eye gets cloudy. It usually happens as we age, as the protein in the lens starts to clump and therefore lose its transparency. The lens has several layers and a cataract can occur in any of those layers. Your optometrist will diagnose a cataract and characterize it by the layer of the lens affected, nuclear, cortical, or subcapsular. There is no identified cause of cataracts up until this point, but correlations have been found between exposure to radiation, UV rays, diabetes, steroids and certain other drugs, or possible even pollution of the air and alcohol. These are statistical correlations, however, and do not provide explanations.
To fully correct a cataract the only option is to remove the lens surgically. This should be considered once the cataract has progressed to the point of serious vision impairment. Up until that point, new and stronger prescriptions can relieve some of the problems, as well as using outside sources like a magnification tool or a certain kind of lighting. Cataract surgery is highly successful in repairing vision once the cataract has progressed to that point.
Sources:
Swanzy, Henry Rosborough. A handbook of the diseases of the eye and their treatment.