A crown is a tooth-shaped cover placed over a tooth that is badly damaged or decayed. Many people call it a cap.
Crowns may be placed for several reasons. Usually the tooth has been broken or severely damaged by decay. As a result, a filling can't replace enough of the tooth or make the tooth strong enough. A crown may hold together parts of a cracked tooth. Crowns can be used to improve appearance as well. They may be placed to cover misshapen or badly discolored teeth.
Crowns usually last at least 7 years. In many cases they last much longer, up to 40 years or so. Maintaining good oral hygiene, including brushing and flossing, will help your crown last longer.