You can change jobs at level 80, though I don't think promotions are implemented in the NA release yet. You don't HAVE to promote, but it's a pretty bad idea not to, seeing as the promoted skills are far more powerful!
"Ebert and Roeper give Tulki two thumbs way up!" "Tulki is fun for the whole family!" ~ The Vancouver Sun
You are the class you initially choose untill you hit 80. unlike maple story, you actually have a ton of skills and weopon choices as you level to 80, so your character will evolve much cooler as you go anyways.
Press K to open up skill window. Then you see Misc, Emoticon, Weapon and one other thing. Perhaps basic skills, not important.
So in Misc, you have general skills like for Warrior it's adding STR or Physical Dmg by small amounts. Then in Weapons, you have options at the bottom to open up sub-tabs of every possible weapon you can equip. So at Lv 2, you get first level up therefore first SP and then put it into either Misc or any Weapon. No Job adv, just choose where to put your SP and for what weapon.
Certain skills need a pre-req skill at a certain level, other skills come at certain levels and cost money to learn. Like I have 3 main attack skills in one skill tree, and one is Lv 1, the other Lv 20, the last is either 40 or 60.
They're known affectionately as Pogoz Girlz. And if you try to pet them Pogo will come around and slam a pneumatic bolt where you thought your balls should be.