The Enhanced Edition brings many elements of Neverwinter Nights up to date, which is pretty much is the same treatment that the studio has given other Black Isle / Bioware classics like Icewind Dale, Planescape: Torment and Baldur's Gate 1 & 2. In this case, that's a UI that scales right up to 4K and advanced graphics options to help spruce up and prettify the game.
Neverwinter Nights: Enhanced Edition is tentatively set at a price of $20 on Steam. No word yet on what the regional pricing will be for PH users, but that price tag will get you both expansions Shadows of Undrentide and Hordes of the Underdark, plus the Kingmaker, Shadowguard and Witch's Wake modules.
Crack out double that price -- i.e. $40 -- for a Deluxe Edition and you'll get the modules Pirates of the Sword Coast, Wyvern Crown of Cormyr and the procedurally generated Infinite Dungeons, which scales characters up to level 40.