Ultrasonic Motion Detectors work by bouncing ultrasonic waves around a room or space, and detecting changes in the contents of the space by changes in the reflected waves that return to the sensor. This type of sensor is difficult to defeat, but is also difficult to program to ignore pet entry into a home area, making it better suited for areas where there is no legitimate traffic which the system must ignore.
Microwave Motion Detectors bounce microwaves around the area, much like the radar guns used by the traffic police. Like ultrasonic systems, they are effective and hard to defeat, but also hard to customize.
Image Recognition Motion Detectors work by assigning a computer to continually process the input from a digital video camera. The computer algorithm constantly scans the pixels of the transmitted image, and triggers an alert when enough pixels change in value, indicating an intruder or major change to the monitored space. This is a high-end solution.
Laser Motion Detectors send a laser beam (or a photoelectric light beam) across an area and detect interruptions to the beam from a sensor located across the space from the beam source. These forms of detectors are somewhat out of date, because they are relatively easy to evade if an intruder knows of their presence.
A motion detector can be wired into the business or home security system and programmed to perform a wide range of tasks in response to an intrusion. The system can call a designated phone number with an alert message, send a signal to a monitoring office (which can then dispatch an investigative team), turn on a light, sound an audible or silent alarm in your home security system.