A crown or toothcap is a covering for a tooth that has lost a great deal of its structure but still has good roots under the gum line.
A dental bridge on the other hand is a system that uses several crowns to replace a missing tooth.
Sometimes a large piece of tooth mass breaks off due to an accident, an old weak filling or tooth decay. The roots of the tooth however may still be healthy. It is best to try and save the natural tooth and prevent it from becoming further damaged. This can often be done with a crown.