An ideal object that is a perfect absorber of light (hence the name since it would appear completely black if it were cold), and also a perfect emitter of light. Light is emitted by solid objects because those objects are composed of atoms and molecules which can emit and absorb light. They emit light because they are wiggling around due to their heat content (thermal energy). So a blackbody emits a certain spectrum of light that depends only on its temperature. The higher the temperature, the more light energy is emitted and the higher the frequency (shorter the wavelength) of the peak of the spectrum.
The Cosmic Background Radiation is an example of a nearly perfect blackbody spectrum.
Two important characteristics of blackbody radiation are described by the following relations:
Wien's Law: This is the law of light that says for blackbody emission, the higher the temperature of the blackbody emitter, the higher the frequency (or shorter the wavelength) of the predominant light it emits. The specific relation is (Peak Wavelength) = 0.29/T where the wavelength is given in centimeters and T in degrees Kelvin.
Stefan-Boltzmann Law: This is a law of blackbody radiation that states that the amount of energy given off by a blackbody per second per unit area (see flux) is proportional to the fourth power of the temperature of the blackbody. In practical terms this means that hotter blackbodies give off a lot more energy than cooler blackbodies (by the fourth power of the ratio of their temperatures to be exact).