Most things on earth absorb and emit some level of electromagnetic radiation (heat), even rocks that are heated by the sun and then cool in the shade. Living things, like people, emit enormous amounts of this energy because we are warm blooded. The difference between the heat we emit and that of a cold car engine are easy to measure with thermal-camera technology.
Heat, or electromagnetic radiation, consists of photons that travel through the air in waves. A portion of these waves travel in the infrared portion of the electromagnetic spectrum. Most thermal sights contain a sensor, called an uncooled micro-bolometer, which detects those waves of photons traveling in the long-wave portion (8 to 12 microns) of the electromagnetic spectrum. The detector then converts the photons to electronic signals.