http://en.wikipedia.org/wiki/Dental_whitening

Although teeth seem very hard, the enamel has microscopic pores that discolor over time as we age, from smoking, and from staining substances that we eat and drink. Fortunately, there is an easy, painless way to make your teeth whiter with bleaching – a dental technique that can give you a bright smile! Bleaching simply removes stains from the enamel of your teeth. In some cases, enamel discoloration occurs during early tooth formation from antibiotic and fluoride consumption. In those cases Bleaching may not be helpfull.

http://en.wikipedia.org/wiki/Veneer_(dentistry)

The development of new adhesion techniques and materials that enable us to successfully adhere porcelain to teeth. Porcelain laminate veneers are shells of ceramic material that can be chemically bonded to tooth structure and they are an aesthetic and safe way to treat permanent stains, teeth that are crooked, disproportionate and structurally damaged.