PIR sensors are used to sense movement of people, animals, or other objects. They are commonly used in burglar alarms and automatically-activated lighting systems. They are commonly called simply "PIR", or sometimes "PID", for "passive infrared detector".
These sensors can be used with microcontroller boards like Arduino. The digital output of the sensor stays at logic 0 when there is no movement. When the sensor detects movement, the digital output goes to logic 1.
Two trimpots placed on the sensor board named Sx and Tx are used to adjust the senstivity of the sensor. Sx pot adjusts the detection range between 3 and 5 meters and Tx pot adjusts the duration of the logic 1 state after the motion is detected.