Infrared light is electromagnetic radiation with a wavelength between 750 nanometers and 1 millimeter. Human eyes are not equipped to detect radiation with wavelengths in this range, although some animals -- especially some snakes -- can detect infrared.
Infrared light consists of alternating electric and magnetic fields at right angles to each other propagating through space. Like all electromagnetic radiation, it's a form of energy and as such can be emitted, absorbed, reflected or scattered by matter it encounters. The greater the wavelength of a photon of infrared light, the lower its energy.
Warm objects like humans and space heaters radiate light in the infrared region of the spectrum. The warmer the object becomes, the more energy it radiates and the shorter the peak wavelength of the radiation it emits. When you feel the warmth from a fire or a heating element, part of what you are feeling is the infrared radiation emitted by the hot fire.
Scientists use infrared radiation for many purposes, including the study of celestial objects with infrared telescopes and the characterization of organic compounds with infrared spectroscopy. Night vision goggles and cameras detect activity in the darkness by using infrared light emitted from warm objects. Some animals like pit vipers can detect warm-blooded prey by sensing the infrared light their prey emits.