dlc and expansions are the same thing, people attempt to divide dlc into hearthfire categories and the larger dlc into expansions. But they all expand the game, and they are all downloadable so i don't see a difference
Personally I feel as if expansions offer a lot more content such as Bloodmoon or Dragonborn. There is also things like Sims 3 Ambitions and WoWs Cataclysm are also considered expansions.
Dawnguard and Knights of the Nine was just DLC in terms of content added compared too the items listed above. All these was were questlines with a few odds and ends nothing more.
As for what bethesda is actually going to do we have no idea as nothing has been announced by the company.
i think if i had to guess i'd say all the stuff besides house mods were expansions, dlc makes me think of hearthfire and such. The little stuff like armor packs in other games would be dlc because they don't expand the world any (i guess the houses add to the world, but they don't add any story)