That's what turn the lights on and off. Can be large. If you look at the home flourescent lighting, you'll see a small round metal thing. That's it. But aquarium lighting requires larger ones.
Retrofit is a term used to describe tnings that are built after the fact. In other words, you can get contained light kits. Got the legs, fans, ballast, and light fixtures all mounted and ready to use. Or, you get a all those peices individually and put'm together yourself. Maybe in a empty hood, or hanging from the ceiling. Google on SW
aqurium lighting and look at the various lite kits/fixtures. I think you'll be able to see the dif there.