I'd say

-Normal 8 man/Alliance Raid/Dungeons/Trials/Duties/Guildhest all slot into the same difficulty assuming you have the correct ilv. Basically tourist mode intended for a guaranteed clear for all players.

then a massive jump with no in-between.

-Extreme trials
-Savage
-Ultimate

PotD/HoH have their own seperate category where the difficulty scales depending on floor.