All gas bulbs have a ballast. Fluorescents, HID, even the new plasma lights have a ballast. Most fluorescents don't get so hot due to their lower power so they work fine with a self-enclosed ballast. HID lamps have their ballast away from the bulb, because the bulb gets hot and so does the ballast. This makes for loads of heat output.
Basic setup of any gas lamp is power cord to ballast, ballast to bulb.
I use T5HO fluorescents because of the great quality of light and output for power used. Watt for watt, a T5HO at 54watts does 5000 lumens, most of these 50 watt MH lamps I'm looking at are doing about 3400 lumens. T5HO - 93 l/w versus the MH 68 l/w. The CRI and PUR/PAR is better on the T5HO, as well. Until you get to over 250w of power, the MH doesn't compare to the T5HO. My setup using T5HO for veg is awesome. I have nice bushes and it's only week three since I picked up and transplanted, they are almost two feet tall. I'm going to flower after the next week, then I'll kick on the HPS above. Right now, the lights are mounted sideways to keep the entire plant well-lit while they grow upwards for their last week.