There are two commonly available types of "Motion Detectors" available today: "Passive Infrared Detectors", "Microwave Motion Detectors", and Duel-Tec Motion Detectors.
Passive Infrared Detectors
By far the most common device used for detecting motion within a building. They work by detecting changes in the infrared light level within their field of view. While the process can be somewhat hard to explain, it is best compared to the old infrared lights we use to see at restaurants over the hamburgers; while the light energy itself is not hot, when it is absorbed by the burgers it converts to heat energy. The infrared motion detector works the same way - all living bodies emit infrared light and through the lens of the detector, this light is focused onto a "thermo-electric" sensor which detects the change in temperature. (Infrared light is also what enables you to "feel" a person silently sneaking up behind you - sorry, it's not magic - just the back of your neck heating up!) Most infrared detectors use at least two different thermo-electric sensors to allow them to distinguish between a person moving in the area and just the room warming up. Advantages: Low cost per unit, wide ranges of coverage patterns available with different units or lens assemblies (typically 25' to 200' width&depth of effective range), Effective for detecting unusual methods of entry, i.e. attic accesses, roof hatches, etc., also effective for detecting persons who hide in businesses until after closing, good at ignoring moving inanimate objects, i.e. ceiling fans, falling items, etc.. Disadvantages: Detect good guys as well as bad - so can't be turned on while an authorized person is in the field of view, Can be activated by very bright lights, i.e. headlights - halogen flashlights - nearby flashes from lightning, etc.
Microwave motion detectors
The second most common method of detecting motion within a protected area. They detect motion by emitting a very low powered stream of microwave energy which is reflected back to the detector by non-moving objects at the same frequency as was emitted and by a slightly higher or lower frequency by a moving object or person. The change in frequency of the energy that is reflected back to the detector is known as the "Doppler Shift". This is the same as when a car is approaching you while sounding it's horn - as it approaches, the sound is at a higher pitch than it would be when standing still&as it passes the sound is at a lower pitch. While microwaves are not heard, the same law applies - the frequency changes when a person moves towards or away from the detector. Advantages: can actually be placed behind a wall or ceiling and still detect movement in the area, relatively low cost per unit, highly adjustable effective ranges, Effective for detecting unusual methods of entry, i.e. attic accesses, roof hatches, etc., also effective for detecting persons who hide in businesses until after closing. Disadvantages: Can be activated by inanimate moving objects, i.e. ceiling fans, water moving in plastic pipes, objects outside of the protected area. Range can be affected by humidity levels.
Dual-Tec motion detectors
They are rapidly becoming the method of choice by many alarm companies. They offer the advantages of both passive infraredµwave motion detectors in a single unit. High detection ratio with very few of the accidental or false alarm problems. Each of the detection methods must detect motion simultaneously in order to activate an alarm. Example - The microwave detects motion, which is actually a large truck moving just outside the protected building - since the passive infrared detector does not normally detect inanimate objects, no alarm will occur.