There are no classes, there are Careers. The difference is, you stay in a Career for certain levels, but eventually you'll probably switch to a different career.
Example of progression: burglar up to level 5, then maybe you switch to Hitman for 4 levels, and then you manage to unlock the Assassin advanced Career and stay there for level 10+.
Or maybe you start as an Apprentice, later on a mage, but decide to not pursue magic anymore with that character and switch to mercenary, ending up as an interesting hybrid character.
You may pick your initial career, but getting it randomly will grant slightly better initial stats. So you may end up a beggar... But who knows, one day the beggar can be a mighty knight, and the beggar's ability to resist disease can save his life.
In total there are around 60 Careers in Archaelund, half of them basic and the other half advanced, which means they have requisites.