A magnetic switch can be mounted on e.g. doors, windows, gates or pet flaps.
Inside the switch there are 2 magnetizable, flexible, metal reeds. A magnetic field will cause these to come together, thus completing an electrical circuit. When the circuit is broken the switch will send a signal to the alarm system or access control system. Once the magnetic field closes again and the electrical circuit is complete again, the magnetic switch will send a new signal telling the system to reset.
Some people also call these switches reed switches or proximity switches.
A magnetic switch is usually combined with some form of sound or light signal so that the emitted signal is visualized.