I imagine there are multiple kinds, so just list them all.
Some of them work by detecting the doppler shift of the reflected energy from a moving object. I’ve found that many of them will not trigger if you move very slowly. This can be annoying if they are used to control the lights in a room and the lighting controller assumes nobody is in the room because you are quietly reading or studying.
The “how stuff works” page doesn’t give the whole story on PIR (passive infrared) detectors commonly used for alarm systems and security lighting.
The key to understanding these is the lens array on the front. This has a couple of dozen or so Fresnel lenses integrated into it, in a two demensional array.
Each of these lenses forms an image of the area under surviellence. So if you have one warm body in that area, there will be a couple dozen hot spots down at the image plane. As that warm body moves, all those hot spots move around the image plane.
There is ONE simple, non-imaging pyroelectric detector near the center of the image plane. As the warm body moves, the motion is converted to a time varying signal at the detector.