HID lights require ballasts to operate, which regulate and provide electricity to the bulbs.
Certain ballasts power only MH bulbs, some only HPS bulbs and others are switchable and can power either. Ballasts require either 120 volt or 240 volt power.
There are two types of ballasts, magnetic and digital. Magnetic ballasts are the original type and are heavier than digital ballasts. Digital ballasts usually offer a dimming feature.