When the portion of the tooth covered by enamel becomes so damaged from decay or trauma that it must be replaced, the dentist prepares a restoration known as a “crown.” The first step involves tapering the outside edges of the damaged tooth into the form of a peg (or post, if root-canal treatment is necessary) that will later accept the overlying “cap.” Then, impressions of the tooth and the teeth immediately adjacent to it are prepared, from which the lab will construct the replacement crown. The crown may be fabricated of gold alloy, metal covered with porcelain veneer, or resin. During the second visit to the dentist, the temporary crown is removed and the permanent crown is cemented into place. 

P.S. Even though a tooth is crowned, it still must be flossed and brushed along with the rest of the natural teeth.