Short answers, pulled from the story.
Infrared is electromagnetic radiation with wavelengths longer than visible light but shorter than microwaves, generally understood to span from around 780 nanometres to 1 millimetre. It is invisible to the human eye and carries energy and momentum, behaving as both a wave and a particle, the photon.
The discovery of infrared is ascribed to the astronomer Sir William Herschel, who published his results before the Royal Society of London in 1800. He refracted sunlight with a prism and detected the rays beyond red through an increase in temperature on a thermometer, calling them "Calorific Rays".
Infrared is popularly known as heat radiation because objects near room temperature emit most of their thermal radiation in the infrared band. The association is a coincidence based on typical low temperatures near Earth's surface, since light of any frequency heats surfaces that absorb it.
Slightly more than half of the Sun's energy arrives on Earth as infrared. At zenith, sunlight delivers just over 1 kilowatt per square metre at sea level, of which 527 watts is infrared, 445 watts is visible light, and 32 watts is ultraviolet.
Infrared is used in night vision, thermography, infrared astronomy, missile homing, heating, short-range communication, spectroscopy, and weather forecasting. It also reveals hidden layers beneath paintings through reflectography and helps identify molecules by the vibration of their chemical bonds.
Pit vipers, pythons, some boas, the common vampire bat Desmodus rotundus, jewel beetles, and certain darkly pigmented butterflies have thermoreceptive organs that detect infrared. Near-infrared sensation has also been reported in the common carp and in three cichlid species.