A veneer is like a false fingernail, but instead of enhancing the top surface of a finger, a veneer is bonded onto the front surface of a tooth.
Preferentially, veneers are attached on to the enamel (outside) surface of a tooth. Full preparation of a tooth is not needed, and in many cases, no preparation at all is needed. This rule is immediately broken—as are many things in dentistry—due to the realities of a particular clinical need. For example, to help restore damage caused by bulimic erosion of the enamel of the back surface of the upper teeth, this rule has to be set aside, as there may simply not be any enamel remaining for the veneer to bond. So dentin bonding can still step in to help to protect that area, with the newer adhesive materials that are now available.