Infrared light includes a range of wavelengths, spanning from one millimeter down to 750 nanometers. The top end of wavelengths include those used for communication, and the lower ranges are those used for visible light.
There are various wavelengths of radiation waves found on the electromagnetic spectrum, and infrared light is found between the visible and microwave sections. Infrared light waves are generally only visible in forms such as fire, as infrared is an indication of heat coming from an organism. Since humans emit some heat but not as much as fire, the infrared waves given off by humans are not visible to the naked eye.