A reed switch is a none contact electronic switch that opens and closes as a magnet moves in and out of proximity. They are great for protecting opening zones where a conventional plunger switch would not be practical. Common uses are Ute canopy windows, Ute hard lids, and camper van doors.
The Magnet is generally fitted to the item that moves and the switch to the vehicle. If the magnetic contact is broken (magnet moves out of range) then the alarm triggers.
The are other uses, for example I have fitted one into the glove box of both the Obsessive Wagon and the Honda Beat so it turns on a LED to light up when open. As the magnet works through plastic it is totally hidden.
The following photos show one I installed on a Hard Lid of a Toyota Hilux recently:
Being a builders vehicle I knew there was a danger of the switch getting knocked off so made up a custom metal bracket to protect if from abuse.
This video shows it in action. The alarm is a Cobra AK4218.
Autowatch make a wireless reed switch that works with some of their alarms.