Trojan: A huge strenght and dual wield make this class so powerfull so it can kill what ever it want, but they can fail by fighting against a Firetaois cuz of hes very low magic defence. Use a blade or a sword and increase the chanse to kill the Firetaoist by using a blade skill called FastBlade or the sword skill called ScentSword. The firetaoist is magic which allows them to hit from distance while trojan are melee. But this class is a PvP (Player versus Player) class if you like to kill other players.
Warrior: A huge defence and he can use a shield for extra defence, Low magic defence but huge defence make it possible to block huge mobs and will survive in a fight against another player. This class can not use dual wield, only a shield and a weapon.
WaterTaoist: A very important class when you are in wars like GuildWar & PKarena etc. He can revive a player that is dead and have the special ability to heal another player while he is fighting, Boost hes attack and give him more defence by using the skill Stigma and MagicShield.
FireTaoist: A very strong magic class which almost everyone fear. With their skill Tornado and FireMeteor they become almost invincible and just a perfect hit will kill him, Its a class for PvP like Trojans.
Archer: A class which the most peoples think is annoying cuz it can use their skill Fly which allows you to fly 45, 60 or 100 seconds and you can't hit him while he is in air, only archer can attack from air-ground and from ground-air. They can also use the skill Scatter which allows you to kill all surrounded mobs which makes him a good hunter while hunting money and items. This is not an good PvP class, just a good hunting class.
Ninja: Well since i have quit ConquerOnline i don't know that very much about this class but they are very special from the other classes since their is'nt any spells like the other classes since the weapons are different and so on.. But they are cool i think :p