A contact lens (contact) is a corrective lens that fit directly onto the eye to improve vision. As opposed to eye glasses, which sit in front of the eyes, contact lenses are in 'contact' with the eye.
Contacts are sorted into two general categories: hard and soft. Hard contact lenses are rigid and made from glass while soft lenses are made of plastic, generally pHEMA, although some newer lenses use more oxygen permeable plastics. Contacts may be intended for extended use or disposable after as little as one day. Some contacts are designed to block UV light by incorporating UV blocking substances such as benzophenone into the plastic during manufacturing. While most are symmetrical, contacts can be made non spherical to correct astigmatism - the irregular shape of the contact balances out the irregular shape of the eye.
Contacts are normally cleaned with a sterile saline solution. There are several other cleaning methods, including "no rub" solutions that clean the contacts with chemicals or protein absorbing polymers, and putting the lenses in a special case and adding a hydrogen peroxide solution that bubbles oxygen gas over the lenses to clean and disinfect them.
Compared to eye glasses, contact lenses offer a more consistent field of vision because the lens moves with the eye. The field of vision with glasses varies from side to side, distorting peripheral vision. Glasses also only cover a limited area in front if the eyes, further reducing peripheral vision where they offer no correction.