A Crown/Cap is a covering on the tooth which may be metallic or tooth coloured. It may be required when the tooth is terribly broken due to a decay, a fracture or other reasons. After a root canal therapy your tooth structure needs to be strengthened by capping it.

A bridge on the other hand is an extended form of a crown used to replace the missing teeth. It is a cheaper option than dental implants, but needs support on both the sides of your natural teeth, which thus have to be trimmed to make room for the crowns on either side.