PIR (Passive Infrared Sensor)
PIR's are cheap little things that combined with a little chip (PIR Sensor Control IC) and possible a Fresnel Lens will tell you if there is motion in font of it. Especially that of a human body, as it is reacting on Infra Red.
You can not use a PIR for anything but on something that stands still, to detect something else that moves.
No, you cannot use a PIR to make a robot follow you. The PIR only detects changes. A moving robot (even turning it's head) would be one constant "alarm" of "changes" - not possible to tell what is what.
If you want to make a robot follow you, you either have to use a camera and some serious calculations, or something like the devantech Thermopile array sensors or hack an Infrared thermometer (often refered to as a Laser thermometer asthey often come with a guiding laser-beam).. or something else :D
PIR's can only detect changes! A moving robot will detect changes all the time!