Dental crowns are tooth-shaped "caps" that are placed right over existing teeth. In essence, a dental cap becomes a tooth's completely new outer surface. 

Dental crowns help to restore a tooth's shape, size and strength: Since dental crowns completely cover your existing tooth, they restore your tooth's shape, repairing a broken tooth, covering dental implants or holding a dental bridge in place. Dental crowns can also help hold weak teeth together so they can withstand chewing and biting. If your tooth is weak from tooth decay, a root canal infection or if you have a cracked tooth, a dental crown may help. 

Dental crowns are also used to improve a tooth's appearance: Although teeth whitening, dental bonding and veneers can help improve the look of your teeth, dental crowns may be appropriate if a discolored or misshapen tooth is also broken or severely decayed.