A dental crown is a synthetic cap, usually made of a material like porcelain, placed on the top of a tooth.
A crown is typically used to restore a tooth’s function and appearance following a restorative procedure such as a root canal. Removing large portions of a tooth is required when decay is extensive. Crowns are often used to restore the tooth.
Dental crowns are also used to attach bridges, cover implants, and prevent a broken tooth from becoming worse. A crown can also serve an aesthetic use when restoring a discolored or stained tooth to its natural appearance.
More info via the ADA.
A tooth must usually be reduced in size to accommodate a dental crown. A cast is made of the existing tooth and an impression is made. The impression is sent to a special lab, which manufactures a custom-designed crown. In some cases, a temporary crown is applied until the permanent crown is ready. Permanent crowns are cemented in place.
Crowns are sometimes confused with veneers, but they are quite different. Veneers are typically applied only to relatively small areas.