What is a cataract and what is the cause?

The eye uses a transparent lens over the pupil with which to focus etc. A cataract is where the lens becomes cloudy and opaque (not see-through). This leads to blurred vision and if left untreated, blindness.
From my A-level studies, I know that prolonged exposure to UV radiation increases the chances of developing a cataract- which is why sunglasses often have UVA/UVB ray filters in.
Cataracts can be spotted by opticians by modern methods and equipment. The cataract can be easily removed (my grandma has just had hers removed). A small cut is made in the eye (after anaesthetic etc!). The cataract is removed with a rod, while a new synthetic lens is placed into the eye. The operation takes around 30 mins and has extremely high success rates. See the NHS website for more details.