Both crowns and most bridges are fixed prosthetic devices. Unlike removable devices such as dentures, which you can take out and clean daily, crowns and bridges are cemented onto existing teeth or implants, and can only be removed by a dentist.
A bridge is a solution for a missing tooth where the teeth either side are prepared for crowns and a replacement tooth is bonded in between them to fill the gap. The use of dental implants means that there is now less of a need for these restorations.
The final crown is made out of a biocompatible metal, ceramic or a combination of both.