HID lighting stands for High Intensity Discharge, which is a type of light that is one of the more intense (brighter) types of of lighting available. An HID lighting system consists of a ballast, reflector, socket and lamp (light bulb). The ballast acts like the engine, converting and driving energy to illuminate the lamp.
HID lighting options include High Pressure Sodium (HPS), Metal Halide (MH), Mercury Vapor and Low Pressure Sodium. The two typically used for plant growth are HPS and MH systems.
HID systems, specifically 1,000 watt HPS lights have been the standard for greenhouse and indoor lighting for decades.  Only in the past couple of years they have started to be replaced with more efficient and targeted LED lighting systems.