A veneer is a thin covering that is placed over the front (visible) part of the tooth. Veneers may be used to lighten front teeth that are naturally yellow or have a gray cast that cant be whitened by bleaching.
Cavities are holes in the tooth which is a result from acid attack. This process gradually starts in the enamel and can possibly spread to the softer inner layer of the tooth. Decay is caused by food that has been trapped in between the teeth.