HID, what is it? Firstly, HID stands for High Intensity Discharge. In easy terms, its a type of light bulb, a very large light bulb (some about the size of your forearm) Its a sealed tube filled with gas, and creates light when the gas is ignited. A regular household light bulb is similar. A regular light bulb has a small metal filament inside that lights up the gas, but in an HID bulb it has two electrodes that arc electric current between them to light up the gas. (think old school mad scientist laboratory).
HID bulbs are categories by the type of gas used in the bulb. The primary types used in horticulture include metal halide (MH), and high pressure sodium (HPS). HID lighting systems consists of three main components, a ballast used to power the HID bulb, a reflector or hood to maximize the bulbs efficiency (by directing light over your plant surface), and a bulb.