PIR Sensor（Passive Infrared Sensor)
Every heating element will produce Infrared Rays and the strength of Infrared Ray's radiation of one object is related to its temperature. The higher the surface temperature, the stronger the radiation energy.
Passive Infrared Sensor of Reolink cameras can detect the change of Infrared Rays radiation energy of the surrounding environment automatically to realize the alarm function.
You may refer to What is PIR Motion sensor for further information.
Motion Detection is the way that Reolink PoE cameras and Wi-Fi cameras can realize alarm function. When cameras detect that is a movement going on the monitoring screen, they will start to alarm. In other words, when a Reolink PoE cameras or Wi-Fi camera detects there is a motion on the deployed area, it will start to alert.