A crown, or "cap," is used when a tooth breaks or decay becomes too extensive to repair with a more simple type of restoration. Sometimes a crown is recommended for a tooth that has had a root canal treatment.
Crowns provide extra strength and support to a tooth that may otherwise fracture apart while being used for typical functions such as eating and chewing. When a tooth is lost, an implant is definitely the best way to replace it. Implants are also restored with a crown.
These crowns are matched to the shade of the surrounding teeth. When a tooth is missing, it can also be replaced by placing a crown on both sides of the missing tooth and with an attached fake tooth in the middle. This is called a bridge and functions similarly to a crown.