Posts from Class
1) Basic, which is the starting class, and you don't even need to acquire it.
2) Advanced, such as various Crusaders. To get this, you need to complete a quest. This, of course, gives certain advantages, such as new skills to learn, increased health, and mana.
3) Dark/Light. To get this, you need to complete a quest for the advanced class and choose to side with darkness or light (later in the story, messengers will come from Bracada and Deyia; I think it's clear who represents light and who represents darkness). After that, you can take a quest from the trainers to become, for example, a Black Knight. This again gives new advantages and access to new skills (for example, a sorcerer who becomes a Lich can learn dark magic, as can a Dark Priest).
I have a few more questions:
1) Is there only one advanced class for each character? Will it affect the third class?
2) There are also quests, for example, for a Druid... although I don't have one in my team... can these quests still be completed, but the profession won't increase? Or can they not be completed?