Tanks have high p.def/m.def output and low damage output. Tanks also have skills that improve their defence abillityes like UD/AI/Majesty or their shield degree. Tanks can use stuns with high base land rate , and they can use aggression/aura of hate(hate aura)/shackle. Play with them and see the differences.
On the other hand , gladiator have high dmg output. Even if he wear heavy armor, it's not a tank class. Or destro too.
BD/SWS are knight classes like tanks , but they are support classes (assistants) because they have dances/songs. They also have arrest/aggresion/UD. Dwarfs are support classes too, they craft and they spoil.