The eye is very much like a camera in that they both have a lens that focuses an image onto film.
In the eye, the lens is focused using muscles, and the film consists of a thin membrane of nerves that line the inner back of the eye, called the retina. If the lens of your camera were to be smudged or scratched, your pictures would be blurred.
Similarly, when the lens in your eye becomes cloudy, light cannot pass through easily, and you see the world as blurred or distorted. The term cataract therefore is used to describe a natural lens which has turned cloudy, usually as part of the natural aging process.