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.
The Druid also casts spells for less MP, much like the Sage, and have access to a far greater pool of spells and skills. In order to become a Druid, one must master both the Sage and Luminary vocations.
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.