A cataract is a clouding of the normally clear lens of the eye.  The eye is like a camera.  In a camera, a lens focuses light onto the film.  Similarly, in the eye a clear lens focuses light onto the retina.  If you were to smear the lens of a camera with grease and then take a picture, the picture would come out blurry.  Likewise, if the normally crystal-clear lens of the eye becomes cloudy, the "pictures" the eye sees will also be blurry. 

What Causes Cataracts?  

A cataract is most often related to aging of the eye.  A family history of cataracts can play a role in the development of cataracts, as can a history of diabetes, eye injury, or use of certain medications (most notably steroids).  Frequent and extensive exposure to ultraviolet light has also been shown to hasten the development of cataracts.  No specific evidence links diet to the development of cataracts.