The ballast is the component of a High Intesity Discharge (HID) lighting system that draws power from the power source and ignites the bulb. There two different types of ballast, Digital and Magnetic. Digital ballast have better effciency and run both HPS & MH bulbs and have the ability to run the bulb at reduced power (dimming). Magnetic are older technology that usualy run only one type of bulb, HPS or MH and are less efficient the there digital cousin.