Luminescence can be described as the emission of light by certain materials which do NOT result from heating
(that is, the emission of light is when the temperature is below that of incandescence). Luminescence is the basic principle behind the working of luminometers.
This phenomenon is usually ascribed to oxidative reactions which take place in solution producing molecules in an excited state.
Some of these reactions release energy in the form of heat while others release in the form of photons.
Examples of luminescent compounds are luciferin (light emitting compound found in organisms), luminol (chemical exhibiting luminescence).
There are two major categories of luminescence as chemiluminescence and bioluminescence. It is easy to understand them as the name itself suggests the meaning. So, chemiluminescence is the luminescence produced by some chemical means. For example, luminol when oxidized with hydrogen peroxide (H2O2) in the presence of a catalyst produces luminescence which is called the chemiluminescence. On the other hand, luminescence which is produced by the interference of an enzyme is referred to as bioluminescence.