How do you like your eggs! Choosing Rear or Surround Channel speakers can have just as many options! On-Wall and In-Ceiling are the two most popular choices and even within these categories there are variations. Two is the minimum as in a 5.1 set-up, but it is possible to use more as in a 7.1 or even 11.2 set-up! The primary purpose served is to deliver ambience and diffuse sounds in a film or music soundtrack.
Scroll down the page for On-Wall types. You may also be interested in Satellites - In-Ceiling - In-Wall