Fixed Prosthesis are general term to use to define Crown-Bridge. They are
described as "fixed" because there is no need to remove the prosthesis by the patients They are  made  to replace  missing teeth when there is a few missing teeth, or  restorate the aesthetic and funtion of the  teeth ,when it has loss of substance .
Fixed Prosthesis divides into categories;
   -Metal
   -Metal based ceramic (porcelain)
   -Zirconium based ceramic
   - Full ceramic

Crown: This is a kind of fixed prosthesis made on a single tooth.
Bridge: This type of fixed prosthesis is used  to replace  the  missing teeth with support of the natural teeth on the  both sides of missing area.

Crowns and bridges, are based on covering of the  teeth with suitable material. after making them smaller.