Any object emits. This emission follows approximately a physical formula called Planck's law, which describes the intensity of the emission of a black body (or in other words, a perfect emittor). This Black Body radiation depends mainly on the temperature of the body (on the right, the intensity versus wavelength for 3 black body radiators at temperatures of 4000K, 6000K and 8000K).
The peak of the emission shifts to shorter wavelengths as the temperature is increased. The radiation from the hottest black body peaks in the blue range; while the coolest black body peaks in the red range of the visible spectrum. The wavelength at which the emission is maximum, is given by the Wien law. The total luminosity (given by the area under the curves) is proportional to the 4th power of the temperature by the Stefan-Boltzmann law.

An interesting example of a black body radiation is the thermal emission of the Earth (or any other body). This thermal emission (also called infrared emission, due to its characteristic wavelength) is due to the Earth's temperature. In the picture, we can see the real emission compared to a black body radiation of a body at a temperature of 280K. In the picture it is also possible to see the absorption spectral lines of oxygen and CO2.