The electromagnetic spectrum is made up of gamma rays, x-rays, ultraviolet light, visible light, infrared radiation, microwaves, TV, and radio signals. The difference between infrared radiation, visible light, and other parts of the electromagnetic spectrum is simply the wavelength of the energy. A micron is one millionth of a meter. Infrared cameras operate in one of two transmission windows in the atmosphere. The NightSight family of products are long wave infrared cameras.
All objects emit infrared radiation.
The amount of radiation emitted is proportional to the temperature of the object. For the most part, our eyes work by seeing visible light reflected off of objects. Conversely, infrared cameras primarily see infrared energy emitted by objects. Examples might include a log in the water or a shiny metal gun sitting in a field overnight. In other words, a shiny metal surface reflects most of the infrared energy that strikes it, but the material surrounding it may be absorbing and re-radiating most of its energy thus appearing to be a different temperature.