Post release content:
1) If you didn't choose to download it and can't disable out of it explicitly, it's free, and it was made by the game developers, it's a patch
2) If you chose to download it, you can disable it explicitly, and it was made by the game developers, it's DLC. You may or may not pay money for it.
3) If you chose to download it, you can disable it, it's free, and it was made by someone other than the game developers, it's a mod.
4) If you chose to download it, you can disable it, you paid for it, and it was made by someone other than the game developers,
That's Illegal! (and no longer protected under fair use or transformative work laws)
