Sometimes a fracture or other damage to a tooth is so large or has weakened the teeth, that a filling cannot repair it adequately. In these cases to prevent further deterioration and to restore the tooth quality, the healthy remainder of the tooth can be made whole again by fitting a crown on it. Tooth decay can be from a variety of causes such as; cavity, fracture, or a combination of both. A crown serves to restore the original shape and function of the tooth.
There are different types of crowns: 

Porcelain metal
Pure porcelain
All metal crowns (usually gold)

These kinds differ in durability, strength, appearance, and cost. If you give your crown regular care by brushing and flossing, it should last for many years. Keep in mind that the most susceptible area of the crown is where it meets the tooth at the gum line so make sure not to leave plaque sitting there for any length of time.