In Ragnarok 2 LOTS, classes mean job in first RO. There is 5 basic classes, Swordman, Archer, Thief, Magician, and Acolyte. Each class has special ability and skill. Swordman and Thief are physical fightning, they are call melee. Archer is Range Attack, archer attack with ranged physical attack. Magician use magic to attack, and Acolyte basically is support type. But Acolyte can built to be Acolyte battle if you want. In Noel update will release new two classes, they are Crescentia and Soulmaker.
Swordman is melee fighter, great in physical attack with high defend and medium attack. Almost people need this classes to tank enemy. His defend is absolutely needed. When you reach level 25 you can change to Knight or Warrior Class. Knight has great defend and low Attack, but if you want balance class you can choose Warrior. Warrior has high def and high attack. But Knihgt is better at tanking enemy especially MVP boss.
Archer is long ranged fighter with high dps (damage per-second). This class is needed when you want fast hunt and fast killer on your party. Archer can become Ranger or BeastMaster when you reach level 25. Ranger dps is the best with fast and high damage. Beastmaster has high HP (Hit Point), but his defend is medium not too good and not too bad. Beastmaster can be your second tanker, not pure tanker.
Magician is magic type. Magician uses intellegence (INT) to attack enemy. Magician has high damage but low defend, so you must keep distance when attack an enemy. This is a needed class in party to kill enemy mobs. Magician can become Elementalist (Sorcerer) or Wizard at level 25. Wizard has high dps for killing monster and MVP, Elementalist also has great dps but elementalyst can heal party area. This skill is very usefull for MVP and raid.
Acolyte is like magician, strong with intellegence and has low defend. Acolyte is support type that can heal single user. But if you reach level 25, you can become Priest or Monk. When reach level 25 your character will automatic reset status and skill. Because if you are became Monk, you can not use heal. Priest has very important support role in party, buff like Blessing and Heal is needed. But Priest can became dps type because priest has magical skill like Judex and Genesis Ray. If youre became Monk you can be a tank. Monk def is very high. Monk has the best def in this game, because his skill Stone Skill (Steel Body) that increase HP +35% defend +200% and Agro +300%.