Door or Window Surface Contacts are the easiest to install and cost the least.  The sensor is placed onto the "surface" of the frame, and the magnetic contact onto the "surface" of the door or window itself with a strong adhesive backing.  Surface contacts are usually used in commercial and industrial applications, but can also be used in residential installations . However, due to their visibility they are not recomended as a device of choice in a residential setting, purely for esthetic reasons.

Door or Window Recessed Contacts work exactly as a surface contact, but the sensor and magnet are installed "inside" the frame and window or door.  They are completely invisible, and less prone to tampering.  Recessed contacts are used most often in office and residential settings.

Garage/Overhead Door Contacts work exactly the same as window and door contacts, but they are larger, more robust and are placed on the door in a different manner.