The High-Intensity Discharge (HID) family of lamps includes mercury, metal halide and high pressure sodium. On this site, a few other related discharge-type bulbs are also included in this category: low pressure sodium, glow lamps and some pulsed arc bulbs.
HID bulbs produce light from an electrical arc discharge contained in an arc tube inside the bulb envelope. The arc tube has tungsten electrodes that terminate the arc discharge at each end of the tube. The arc tube contains a gas mixture of argon and/or xenon and/or neon depending on the type of HID bulb. There are also metals inside the arc tube. Mercury vapor lamps produce light by exciting mercury atoms. Likewise, high pressure sodium excites sodium atoms. Metal halide, the most common of the HID bulbs, uses sodium and other halide compounds as the basis for exciting atoms to produce light.