I am so confused about maplestory classes!?

Okay, so I used to play maplestory, back when it was just the thief, magician, warrior, and mage classes, YEARS ago. Well i just started up now and I'm real confused. I signed up for a cygnus knight nightwalker because i thought thats just what the thief class had turned into, except i clearly recall that thieves had to go to the dark lord all the time for job advancements. I only went to him once to kill valefor... am i not a thief? or is there a difference between the nightwalkers and thieves? and if im not a thief, then what exactly does a nightwalker do that's different? im so confused.