A Cataract is a clouding that develops in the crystalline lens of the eye obstructing the passage of light. Cataracts typically progress slowly to cause vision loss and are potentially blinding if untreated. The condition usually affects both eyes, but almost always one eye is affected earlier than the other.
There are various causes for cataracts including:
Cataracts can occur in any region of the lens and their position determines how vision is affected. Some cataracts cause greater visual impairment then others. Nowadays ophthalmologists do not leave a cataract until it is “ripe” as is commonly thought. Generally cataracts are removed when they start to cause problems.
For example a jeweller may only have a small cataract, but its effect on his quality of life may be significant as he needs good vision for his work. If you drive a car, it's important that the cataract is not permitted to reduce your visual acuity below the legal limit.