Veneers are usually thin shells of porcelain, not unlike the press-on nails, that can be bonded to the outside surface of the teeth to alter their color, shape, and alignment.
Veneers are used when an alteration is desired that is beyond what bleaching or bonding can achieve. Minor gaps or misalignment can be instantly corrected, but major crowding will require braces.
They can be a conservative alternative to crowns because they require much less tooth preparation. An impression is made because the veneers are fabricated in the lab. Most of the tooth, however, remains untouched.