Also take from Wikipedia: http://en.wikipedia.org/wiki/Tooth_whitening
There are two main methods of gel bleaching--one performed with high-concentration gel, and another with low-concentration agents. High-concentration bleaching can be accomplished either in the dental office, or at home. Performing the procedure at home is accomplished using high-concentration carbamide peroxide which is readily available online or in dental stores, and therefore is more cost-effective than the in-office procedure by many orders of magnitude. Whitening is performed by applying a high concentration of oxidizing agent to the teeth with thin plastic trays, for a short period of time, which produces quick results. The application trays ideally should be well-fitted to retain the bleaching gel, ensuring even and full tooth exposure to the gel. Trays will typically stay on the teeth for about 15-20 minutes. Trays are then removed and the procedure is repeated up to two more times. Most in-office bleaching procedures use a light-cured protective layer that is carefully painted on the gums and papilla (the tips of the gums between the teeth) in order to reduce the risk of chemical burns to the soft tissues. The bleaching agent is either carbamide peroxide, which breaks down in the mouth to form hydrogen peroxide, or hydrogen peroxide itself. The bleaching gel typically contains between 10-30% carbamide peroxide (15% is recommended) which is roughly equivalent to 3-10% hydrogen peroxide concentration.
Low-concentration whitening is far less effective, and is generally only performed at home. Low-concentration whitening involves purchasing a thin mouthguard or strip that holds a relatively low concentration of oxidizing agent next to the teeth for as long as several hours a day for a period of 5 to 14 days. Results can vary, depending on which application is chosen, with some people achieving whiter teeth in a few days, and others seeing very little results or no results at all. Whitening is potentially better at a dentist because the strip or mouth-guard does not completely conform to the shape of the teeth, sometimes leaving the tips of the teeth (near the gumline) unbleached. The bleaching agent is typically less than 10% hydrogen peroxide equivalent so irritation to the soft tissue around teeth is minimized. Dentists as well as some dental laboratories can fabricate custom fitted whitening trays that will greatly improve the results achieved with an over-the-counter whitening method.
A typical course of bleaching can produce dramatic improvements in the cosmetic appearance of most stained teeth however, some stains do not respond to bleaching. Tetracycline staining may require prolonged bleaching, as it takes longer for the bleach to reach the dentine layer. White-spot decalcifications may also be highlighted and become more noticeable. Bleaching is least effective if teeth have white spots, decay or infected gums. It is also least effective when the original tooth color is grayish. Bleaching is most effective with yellow discolored teeth.
Laser bleaching, also known as power bleaching, utilizes light energy to accelerate the process of bleaching in a dental office. Different types of energy can be used in this procedure. The ideal source of energy should be high energy to excite the peroxide molecules without overheating the pulp of the tooth, creating an inflammatory response. The use of an argon laser to safely and effectively speed up the whitening process is ideal for laser bleaching. The use of an argon laser is preferred over the use of an arc lamp (the traditional dental method of light-activated bleaching) or infrared laser because it does not exhibit any of the heat or UV ray emissions of the arc lamp. Chemical burns, which are occasionally a side-effect of gel bleaching, or heat-induced sensitivity, are not a factor with argon laser whitening. Most laser teeth whitening treatments can be done in approximately 1 hour, in a single visit to a dental physician, (depending on the condition of a person’s teeth).