Dental crowns are used to protect and restore teeth that have been broken down heavily, or for teeth which have cracks which cannot be treated by routine restorations such as composite fillings. They are also required after a tooth has needed a root canal treatment. This will then allow you to be able to chew and eat without fear of the tooth breaking.
They cover the entire tooth to replicate the original tooth as much as possible. Crowns can be made from many materials including porcelain (ceramic), zirconia and gold alloy.