Rifled barrels are designed to shoot slugs more accurately than smooth bore barrels.  Generally, if you're hunting with a shotgun using a smooth bore, you'd want to shoot rifled slugs.  Rifled slugs have 'rifling' or lands and grooves, hence causing them to spiral down in the smooth bore when fired.  However, they're not perfect and you'd generally use them when expecting to take shots less than 100 yds.  If you're using a rifled barrel, then you would you sabot slugs or other slugs designed for these barrels.  Keep in mind, there are several different types, Foster, Brenneke, Sabot and Steel slugs are the more typical.  Each slug has it's own pro's and con's, if you're going to use a shotgun for deer, I'd suggest trying out a variety of these to see which one your shotgun likes best.  A smooth bore can shoot any type of slug, it just may not perform very well past 75 yds. or so or less in some cases.  As for using BB, buckshot, or other shells in a rifled barrel, I would'nt do that.  The rifled barrel is designed to shoot slugs, using BB or buckshot may damage the rifling.  Hope this helps, I'm sure it's confusing, but if you know anyone from the North Eastern states like NY, Vermont, Mass., Conn., etc., they'd probably be able to explain it a little better by word of mouth.

Having hunted with shotguns for deer for over 40 years, I can give you my opinion and that's just what it is, MHO.

I have shot slug throwers and smooth bore both.  Rifled barrels should be used for slugs and slugs ONLY.  Firing shot through a rifled barrel will damage the rifling making it inaccurate.  Soft lead rifled slugs may be used in rifled barrels with very good results.  I tend to use Federal's cheaper slugs through my H&R 20 gauge slug thrower and can consistantly get a 2 inch pattern at about 125 yards with no problem.  Yes the slugs are rifled, but the fact that they are soft lead allows the rifling in the barrel to grab the slug better giving it the required spin to make it accurate.

The advantage of the smooth bore is that you can shoot shotshells through it without damaging the barrel, but there is a downside.  You will find that you will compromise accuracy with the smooth bore.  I have an identical H&R 20 gauge smooth bore for comparison.  With rifled slugs, it begins to lose accuracy at about 75 yards, but I can bring down a partridge with it where I can't with the slugthrower.

As far as sabots, I am not a fan of them.  IMHO, sabots are over priced for their performance and I can't see paying three times the price of a rifled slug to get 5 more yards of accuracy.  If I can't wait for the deer to walk that five yards, I shouldn't be out there in the first place.

I believe that "slug guns" are a myth.  I was in a sporting goods store looking for a shotgun for deer hunting when the salesman tried to "sell" me a "slug gun".  It was nothing more than a smooth bore with rifle sights.  This is not a slug gun, but it was advertised as one in the literature.  I may be a backwoods country hick, but even I know better than that.  I took my business to another sports shop where I bought my current rifled slugthrower. Does it make a difference?  It sure made a difference to the 12 point whitetail I shot through the heart with first shot I made while hunting after I bought it. 

It sounds like you understnd the gist of it.  As mentioned, most rifled slug barrels come with rifle style sights.  I would also agree to try different brands and types of slugs to see which would work best for your particular gun.  Some guns are like some woman I have known in my life time, finicky.

My mother always said, "When you want to know something, the only dumb question is the one that is never asked."  Always glad to help other hunters out with information if I can.