All objects the have a temperature above absolute zero (–459.67 °F on the Fahrenheit scale),emit a form of light radiation called infrared. This is not visible to the human eye but is visible to special cameras. In effect, the camera "sees heat" or the difference in heat emitted from one surface to another.
An "Infrared Thermographer" is a person trained in using this equipment and interpreting the results.
Here is a great video that explains all about Infrared: Note: The "night vision camera" she is talking about is in fact an infrared camera.
