Blackbody Radiation

Atoms and molecules emit radiation in the infrared, visible and ultraviolet part of the spectrum. One of the main sources of radiation in this part of the spectrum is the so-called blackbody radiation emitted by objects due to the thermal motion of their atoms and molecules.
If you heat an iron poker in a fire, the poker first gets warm, then begins to glow a dull red, then a bright red or even, orange. At higher temperatures the poker becomes white, like the filaments in an electric light bulb. At still higher temperatures, if the poker did not melt, it would become bluish. The name blackbody radiation is related to the fact that an initially cold, black object emits these colors of light when heated.
There is a well studied relationship between the temperature of an object and the predominant frequency of the blackbody radiation it emits. Basically, the higher the temperature, the higher the frequency. Astronomers use this relationship to determine the temperature of stars from their color. The infrared stars are quite cool, our yellow sun has about the same temperature as the yellow filament in an incandescent lamp, and the blue stars are the hottest.

All objects emit blackbody radiation. You, yourself, are like a small star emitting infrared radiation at a wavelength corresponding to a temperature of 300K. In an infrared photograph taken at night, you would show up distinctly due to this radiation. Infrared photographs are now taken of houses at night to show up hot spots and heat leaks in the house.
Perhaps the most famous example of blackbody radiation is the 3K cosmic background radiation which is the remnant of the big bang which created the universe.