PIR Sensor (passive infrared sensor) is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. They are most often used in PIR Motion detectors.
Passive means this device does not emits or radiates any energy for detection. Every object above absolute zero emits heat, this heat signature is in Infrared wavelength so those energy can not be seen by naked eye, but they can be measured by infrared devices.
Almost all the PIR sensor module available are work same way with very miner differences. A very good tutorial can be found here, how they work and how we can use them in different applications.
1. Operating voltage: 4.5V to 20V DC2. Static power consumption: 65 micro-amps3. Output Logic level : High-3.3V, Low-0V4. Delay Time: adjustable (5-200S +-3%)5. Blocked Time: 2.5 Seconds Default 6. Trigger Jumper: L No Repeat, H Repeated Trigger(Default)7. Sensing range: <1400 , 3-7 meters(Adjustable)8. Operating temperature: -20 -+800 C