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Professional Teeth Whitening Systems whether done At-home or at the dentist office requires a carbamide peroxide solution that bleaches the tooth enamel. The active agent is typically a gel that is put in a custom fit mouth guard. The duration and frequency of the treatment varies based on the individual and the strength of bleaching gel being used.
Review Teeth Whitening

There are several over-the-counter products available. Like other at-home treatments, mouth trays (mouth trays will likely be one-size-fits-all) whitening strips, brush on, and the expensive laser treatments. Be aware, though, of the risks of of tooth whitening systems and vendors. You should review the products and companies before you buy.
Professional Teeth Whitening

Like any off-the-shelf product non-custom kits offer little room for customization. Your mouth trays will likely be one-size-fits-all, or perhaps adjustable, but they will not fit as snugly as a tray custom-made by your dentist. A loose tray creates the risk of bleach leaking onto your gums or down your throat, causing irritation. As well, to lessen the risk of irritation, some products contain less peroxide. While the risk is lower, so too is the whitening effect. Finally, any irritation or reaction will require a trip to your dentist anyway.

Some kits, require that you cast a mold of your teeth, then send this mold away to a lab where a custom-made tray will be created and sent back to you. This option gives the convenience of at-home do-it-yourself teeth whitening without the risk of leakage.

Teeth bleaching products change the color of your teeth by oxidization. The hydrogen peroxide or carbamide peroxide breaks down into water and a radical, or compound constituent. The radical attaches itself to stain molecules and weakens them, allowing oxygen to reach and whiten your tooth enamel.

Note: Thanks to dental find.com/glossary/tooth_whitening.html for help with the article.