Magnetic contact switch is a generic term referring to an electrical switch that is operated by a magnetic field. The absence or presence of the magnetic field, provided by a small magnet, opens and closes the switch. In the alarm industry, they are the most common and the most economical form of protection.
Generally, there are 3 types of magnetic contact; recessed, surface, and mechanically actuated.
The recessed contact shown on the left is the most common in residential applications. The entire switch and magnet are concealed within the fabric of the building and when they are pressed into the 3/8-inch holes drilled for their installation, all that remains visible is a couple of 7/16-inch diameter, slightly raised dots on the esurface. If painted over, they can become very difficult to locate.
A surface mount contact is shown below with its associated magnet. Both the switch and magnet are visible after installation. These contacts are most common in commercial applications and in residential installations where recessed mounting is not an option.
A mechanically actuated switch generally refers to a recessed contact, which requires a physical operation to function. Examples of this are roller, pushbutton, or dome type of contacts. The switch and magnet are integral to the contact and actuated by a physical device (button, dome or roller) that tells the switch the protected item is closed or present. They are somewhat prone to failure or malfunction due to paint or dirt buildup on the mechanism over time.
There are special application contact switches designed for specific installations, including high security, overhead door, and pull apart cord types, but generally, the first three types will solve the majority of installation challenges.
Also note, if you have an older installation, pay attention to the contact switches you have installed, as the original varieties of each were not sealed to prevent corrosion, which has since been addressed by manufacturers.