You will need a reflector or hood for your HID lighting system. The purpose of the reflector is to direct the light towards your plant.
Reflectors come in many shapes and sizes and some have features such as air or water cooling. This is necessary since HID lights run hot. Some reflectors have glass at the bottom or a tube surrounding the bulb to contain the heat. Many also come with ducting ports so you can connect ventilation to further cool the HID lighting system.
The bulb attaches to the reflector, which connects to the ballast.