Crowns (or caps) are used to protect a tooth that has :

• weakened tooth structure from a large filling,
• history of a root canal having been preformed 
• to hold a fake tooth to replace a missing tooth (bridge)
• to change the smile or bite to enhance appearance or function or both

Crowns can be made of

• all porcelain - used in most esthetically demanding areas
• porcelain to metal - used where cosmetic and strength are concerns
• all metal - usually Gold used when strength is most important (2nd molars)