For the sake of those not playing the game: FFXIV A Realm Reborn is to FFXIV (original) what Mass Effect 2 is to Mass Effect. A sequel game in the existing universe with the option of importing your existing character.

EDIT: Actually, it could be said that FFXIV:ARR has even more difference to FFXIV than ME2 does to ME1, because the FFXIV games use different engines while ME1/ME2 use the same one. And BioWare didn't even have to make the engine, which SE did. Which makes the speed of release even more amazing!