Paladins are defensive in nature, shamans are more offensive in nature. Does not mean they have none of the opposit capaibilites.
But it is the usual light vs dark.
Best to think shamans like dark knights, shadow knights, stuff like that but instead of dark abilites, elemental abilities.
But shamans CANNOT TANK worth anything late game. They are at best decent healers when they get chain healing. But their DPS is very nice, up there with rogues and mages.
Paladins can tank, can be a good healer. But then their dps is kinda weak.