It's not a dumb girl question at all. I actually find the entire realm of shotguns a bit confusing as well. Your Mossberg 500 most likely has a smooth bore. You should be able to tell this just by looking at it. As far as slugs go, you want to use the most open choke you have (i.e. not the full). Choke tubes are only for shot. If you have a smooth bore, you want rifled slugs (doesn't make sense, does it?). If you have a rifled bore, you want the sabot slugs.
"Throughout history, it has been the inaction of those who could have acted; the indifference of those who should have known better; the silence of the voice of justice when it mattered most; that has made it possible for evil to triumph."
-Haile Selassie I