Metal halide lighting is a type of HID or high-intensity discharge lighting. It is used extensively by professional growers to provide light to their plants in the earlier vegetative growth stages.
Metal halide (MH) bulbs produce a bluish light that's well suited to the early growth stages of a plant. MH bulbs are often used in conjunction with HPS (high-pressure Sodium) lights, which provides warmer light used in the flowering stage.
Plants that like a lot of light (direct sunlight) such as tomatoes should get around 40 watts of light per square foot. For plants that prefer slightly less sunlight such as lettuce, 25-30 watts per square foot will do.
A reflector is a shiny piece of material that you use to direct the light produced by your bulb onto the plants. This makes more efficient use of the available light.
As I mentioned earlier, metal halide lamps are a type of HID (high-intensity discharge) light. This means they need a so-called "ballast" to control the current they receive. A ballast is an electrical circuit that you'll need as part of your MH lighting set up.
MH lights produce heat. More heat than your plants like. This is why it's really important that you use a fan to blow the heat away from your plants and prevent them from getting damaged.