Crowns can sometimes be called caps. It is considered and full coverage restoration. Crowns are usually made outside the mouth and then cemented into place by covering the whole tooth.
A bridge is like having 3 crowns joined together. The middle crown is often there to replace a missing tooth.
Some of the reasons why a crown may be recommended is where the tooth has undergone root canal treatment. The crown is the final restoration used to seal the tooth and protect from further infections. In this case the crown is important for the success of the root canal treated tooth.
Crowns can be used on heavily broken down teeth. Should a tooth be heavily broken down, the dentist may recommend a crown to provide additional protection and life for the tooth.