Bleaching is a process used to brighten your own natural teeth without any complications. No removal, cutting or etching of your dental enamel takes place.
Tea, coffee, red wine, tobacco, and the natural aging of the teeth turn them darker or stain them over time.
A professional brightening is performed with the aid of active oxygen, which decolorizes the distracting color pigments present in the tooth.
More about Bleaching on http://www.checkdent.com
Source: What is Teeth Bleaching?