Inside your eye, behind the iris and pupil is a lens. In a normal eye, this lens is clear. It helps focus light rays on to the back of the eye (the retina), which sends messages to the brain allowing us to see.
When cataract develops, the lens becomes cloudy and prevents the light rays from passing on to the retina. The picture that the retina receives becomes dull and fuzzy.
Cataract usually forms slowly and most people experience a gradual blurring of vision.