Here's a brief introduction to how HIDs (High Intensity Discharge) growlights create all those wonderful photons for your plants!
High-intensity discharge lamps (HID lamps) produce light by passing an electric current between two tungsten electrodes inside a glass tube containing gases and metal salts. When the electric current is sufficient, the gas acts as a conductor, creating an arc between the two electrodes while the metal salts vaporize due to the intense heat generated by the arc. The electricity ionizes the gaseous metal salts (adds electrons) and, as a result, they intensify the light generated by the initial arc stream. Different mixtures of metal salts will produce different qualities of light.
High-intensity discharge lamps are very efficient. They generate more lumens per watt than either fluorescent or incadescent lamps.