A veneer is a thin layer of material placed over a tooth, either to improve the aesthetics of a tooth or to protect a damage to a tooth's surface. It is made to match the color and shape of your tooth.
With porcelain veneers, the dentist takes an impression of the tooth and sends it to the dental lab for fabrication of the veneer, usually after the front of the tooth has been reduced.
The tooth is prepared for bonding by roughening the front surface with mild etching solution. The veneer can then be bonded to your tooth using a dental bonding cement.