There are two (2) general categories of contact lenses which are soft and rigid gas permeable (RGP). Soft contact lenses, as its name aptly suggests are soft, flexible plastics that allow oxygen to pass through to the cornea. Newer versions include silicone-hydrogels to provide more oxygen for the eyes. RGP lenses are more durable and provide a clearer vision. Although not as initially comfortable as soft contacts, they are much easier to handle and less likely to tear.
Both types may be used for extended wear contact lenses or those used for overnight and continuous wear, but only a few RGP lenses are designed and approved for such purposes.