Infrared is electromagnetic radiation whose wavelength spans the region from about 0.7 to 1000 micrometers (longer than visible radiation, shorter than microwave radiation). Remote sensing instruments work by sensing radiation that is naturally emitted or reflected by the Earth's surface or from the atmosphere, or by sensing signals transmitted from a satellite and reflected back to it. In the visible and near-infrared regions, surface chemical composition, vegetation cover, and biological properties of surface matter can be measured. In the mid-infrared region, geological formations can be detected due to the absorption properties related to the structure of silicates. In the far infrared, emissions from the Earth's atmosphere and surface offer information about atmospheric and surface temperatures and water vapor and other trace constituents in the atmosphere. Since IR data are based on temperatures rather than visible radiation, the data may be obtained day or night.
NASA Earth Observatory

Infrared Radiation (IR) - Energy that is emitted in the form of electromagnetic waves at a wavelength greater than about 750 up to approximately 30,000 nanometers. Although the earth absorbs almost all the IR, UV, and visible radiation the hits it, the surface of the earth emits only the longest wavelength radiation of these three, IR, in any significant amounts. It is this re-emission of IR towards space--and its subsequent absorbance, re-emission and scattering--that contributes to the process that heats the atmosphere. This re-emission and subsequent heating is a part of the greenhouse effect.
[Solid State Communications; v88; 939; 1993]
[Journal of Geophysical Research; v97; 11,513, 1992.] 
Atmospheric Chemistry Glossary