Inside your eye is a lens. This lens allows light that enters the eye to focus on the back of the eye. As we age, the lens inside the eye becomes cloudy. A cloudy lens is called a cataract.
There are many types of cataract. The most common cataract causes a yellowing of the lens and is called a 'nuclear sclerotic' cataract (below).
A mature cataract has a very white appearance, that can sometimes be visible even with the naked eye.