The Druid's magical power is unparalleled by any human, the very forces of nature bowing to their every whim. The tides, the storms, even the cracking fury of the earth itself is under their control. Simply put, the druid has access to every worthwhile spell in the game and has more than enough MP to put them to use.
The Druid also casts spells for less MP, much like the Sage. In order to become a Druid, one must master both the Sage and Luminary vocations. The druid has managed to negate most of the physical stat reductions that afflicted the aforementioned classes, having to endure only a slight cut to HP and strength and even enjoying a faint increase to resilience.
Theurgy is the practice of rituals used to call upon the aid of one or more gods. One who practices theurgy is called a therugist.
Ragnarok comes from Norse mythology. It is the time of a great battle between gods and monsters which will herald the end of the gods themselves and the end of the world itself.