There are currently four main categories of lens:-

Hard Lenses

Hard lenses are made from clinical quality perspex.  Oxygen will not pass through this material and the oxygen demands of the front of the eye (cornea) must  therefore be satisfied by tear exchange behind the lens. Because of this, the build up of wearing times is fairly slow as the cornea adapts to the slight lack of oxygen.  Generally unpopular due to poor oxygen transmission Patients who would benefit from a rigid lens will more likely use gas permeable.

Gas Permeable Lenses

Gas permeable lenses are made from modern plastics which allow oxygen to pass through the lens.  In this way, the problems due to the lack of oxygen associated with hard lenses are virtually eliminated.  Some patients, such as those with astigmatism, may achieve better vision with a rigid lens.

Soft Lenses

Soft lenses are made from hydrophilic plastics which absorb water to become flexible.  It is the water content of the lens that allows the oxygen to pass through.  The higher the water content, the greater the permeability. They require much less adaptation time due to their initial discomfort.  This permits more flexible wearing schedules such as social / weekend wear.  Also available in tinted designs, astigmatic prescriptions and even bifocals.

Silicone Hydrogel

A special type of soft lens with higher oxygen transfer then traditional soft lenses.  Perhaps surprisingly they have low water contents, but their high silicone content allows gases to pass easily.  These new materials are currently only available in a few different designs, but may represent the future of contact lenses.

Pros

Easy to clean

Cons

Lengthy Adaptation

Less Stable

Very Low Oxygen

Pros

Oxygen Permeable
Less Spectacle Blur
Less Adaptation

Cons

Less Stable
Initial Discomfort
No cosmetic tints

Expensive Replacement
Dust can be uncomfortable

Pros

Almost no adaptation
Very stable
Can be used occasionally
Can be coloured
Disposable
Cheaper replacements
Fitted from stock

Cons

Less Durable
Multi step cleaning

Pros

Excellent oxygen permeability

Cons

More expensive than standard soft

Limited lens choice