The part of the tooth that is seen above the gum is called the clinical crown. When not enough of the clinical crown is showing, the gum must be moved up the root to expose more tooth. This is called crown lengthening.
There are two situations in which crown lengthening is commonly performed: to improve appearance, and to allow a dentist better access to decay.
Below is an example of how the procedure works to enhance the esthetics of a smile.
Click here for more information on esthetic crown lengthening from the American Academy of Periodontology Consumer Page.