I don't see the (relevant) difference. DLC is downloadable expansions. Additional content is all good by me however it gets to me.
I think everyone wants a single expansion pack to download with new areas, weapons, missions, main questlines and creatures.... instead of how fallout did it, which was make a bunch of downloadable questlines. I can see why they did it though, fallout DLC had about as much content as an expansion pack and cost the same amount as a new game. Why give us an expansion for 15 bucks when they can draw it out and take 50 bucks from us in total?