There's a difference between a Class and a Job. A class is the basic groups you start out as--Marauder, Archer, etc. Jobs are specialized classes you unlock through leveling certain classes and completing their quests--for example, to get Scholar, you must level Arcanist to 30, and Conjurer to 15 (if the wiki is correct), then complete the corresponding quest at the Arcanist guild.

http://ffxiv.consolegameswiki.com/wiki/Jobs