HID stands for High Intensity Discharge. There are two main types of these bulbs when you talk about modern marijuana growing, the High Pressure Sodium bulb and the Metal Halide bulb. To keep it simple, they work on the same principle as a fluorescent bulb but at higher electrical currents and a few additional additives.
HPS, or High Pressure Sodium, bulbs are the most efficient HID bulbs on the market. This can be used for growth throughout the plants life, however, these bulbs lack the light in the blue spectrum, which is essential for vegetative growth. This is best used during the flowering stage of your weed, its during this stage that the plants crave and thrive in the red spectrum of light. When I grow, I use fluorescent lights during thevegetative stage, then switch to the HPS bulbs during budding. This helps to cut costs while still getting a great harvest.
Metal Halide lamps produce light by passing an arc of electricity through a mixture of gases. The metal halide bulb contains a high-pressure mixture of mercury, metal halides, and argon. A halide is defined as a chemical compound consisting of a halogen and another element, especially a strongly electropositive metal such as sodium or potassium. Salt is a halide. The mixture of halides will affect the nature of light produced, influencing the color, operating temperature, and intensity. (making the light produced more blue or more red for example). The argon gas in the lamp is easily ionized, and facilitates the arc striking across the two electrodes when voltage is applied. The heat generated by the arc then vaporizes the mercury and metal halides, which produce light as the temperature and pressure increases.
Metal Halide bulbs are on the same level as the HPS bulbs. This light is primarily in the blue spectrum of light which is critical to plants during vegetative growth. I say that MH and HPS are on the same level because even though the HPS emits more lumens per watt then the MH bulbs, the MH bulbs are the only light that can produce light as intense as the HPS while in the blue spectrum.
HID bulbs offer the best proven results. An ideal setup would use both of these bulbs. A draw back to these light options is that they can be costly and they always generate heat. They have around a 10,000 hour life.