PIR Sensor "Passive Infrared Sensor" reacts to the temperature change (heat energy) caused by the movement of human body. It requires an unobstructed line-of-sight for accurate detection, if there is an obstruction, such as furniture or partition between the person and the detector, the person cannot be detected.
In general, PIR sensor responds to larger movements (i.e walking) and is less influcenced by the air flow interruption which makes it works optimally in the spaces with high air flow and frequent occupant's movement but without having high ambient temperature.
It is possible for user to perfectly define the detector's detection coverage and the unwanted detection area can be eliminated by using the lens shield. The PIR sensor is suitable for both indoor and outdoor application because it is immune to high air flow. The cost of PIR sensor is less expensive then Microwave and Ultrasonic sensors.