As far as a dental restoration goes, crowns and caps are used synonymously.
Crowns are synthetic caps, usually made of a material like porcelain, placed on the top of a tooth.
Crowns are typically used to restore a tooth's function and appearance following a restorative procedure such as a root canal. When decay in a tooth has become so advanced that large portions of the tooth must be removed, crowns are often used to restore the tooth.
Crowns are also used to attach bridges, cover implants, prevent a cracked tooth from becoming worse, or an existing filling is in jeopardy of becoming loose or dislocated. Crowns also serve an aesthetic use, and are applied when a discolored or stained tooth needs to be restored to its natural appearance.
A tooth is usually reduced in size to accommodate a crown. An impression is taken and a cast is made of the existing teeth. 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.
With proper care, a good quality crown could last many years. It is very important to floss in the area of the crown to avoid excess plaque or collection of debris around the restoration.