CONTACT LENSES (modified from Canadian Ophthalmic Society Website)
What are contact lenses?  Contact lenses are small, curved, thin plastic disks designed to cover the cornea (the clear front covering of the eye including the iris and pupil). They can often be used to correct conditions such as myopia (near-sightedness), hyperopia (far-sightedness), astigmatism (distorted vision) and presbyopia (aging eyes that may need bifocals).
Types of contact lenses  RIGID contact lenses, originally created over 25 years ago, are still used by some people. Made of hard plastic, they are very durable but some brands do not allow oxygen direct access to the cornea. A lack of oxygen over an extended period of time can change the shape of the cornea. Newer rigid lenses combine the hard plastic with other molecules that allow oxygen to pass through to the cornea. These are called RIGID GAS PERMEABLE lenses. Rigid lenses are often the least comfortable choice.
EXTENDED WEAR contact lenses (which may be either soft or rigid) are supposed to be designed to be left in the eye overnight, but Dr. Ing recommends ALL contacts always be removed at night.
SOFT lenses, the choice of most contact lens wearers, are also made of plastic but incorporate water, allowing softness and flexibility.
DISPOSABLE lenses are designed for daily or extended wear and are discarded or replaced each week. These lenses are convenient and may reduce the chance of infection or deposit formation.  Discard the used lens at the recommended time.
Risks     Rigid, nongas-permeable lenses are more likely to scratch the cornea if left in overnight or if used without a proper fit. Because it is more mobile, it may slip and become hidden under the lid.  Rigid gas-permeable lenses may allow more protein build-up than rigid gas-permeable lenses, resulting in discomfort, blurring and intolerance. Special cleaning solutions are required and should not be changed without instructions from your ophthalmologist.
Using a soft daily-wear lens as an extended-wear lens may lead to temporary or even permanent corneal damage.  People who wear their lenses overnight, even with extended-wear lenses have a greater chance of developing corneal infections or a lens overwear syndrome that is commonly confused with infection. 
Caution must be exercised when handling, cleaning and disinfecting these lenses and their cases. At the time when lenses are inserted, the case should be thoroughly rinsed with warm water and allowed to air dry. Home-made saline solutions should not be used as they have been linked to serious eye infections (Acanthamoeba).  Do NOT go swimming or hot-tubbing with your contacts.
Who should not wear contact lenses?  Some of the conditions that may keep you from using contact lenses include:  frequent eye infections, severe allergies, dry eye, a work environment that is very dusty or dirty , inability to handle and care for the lenses properly.
If you are having problems with your contacts:  Remove them immediately, get an eye exam, and bring the contact lens, lens case and solutions to the check-up.  If you have disposable lenses, and do not have a red eye, try a new pair.  If your eye is not red or painful you can try wearing the contact lens less frequently, or add artificial tears.  If you really, really must wear contacts, try wearing just one contact lens in one eye, and wear just one contact in the other eye, the next day.