I can only basically describe what each does, i personally dont have too much experience in these though..
Hunters basically just kill theives and NPC theives, getting exp for doing so.. i think..
Theives kill hunters or run from them xD and rob 2 star and higher traders by taking out their transport, summoning your own, loading it up and taking it back to bandit den and selling the goods for money. Obviously killing anyone that gets in your way

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Traders basically buy goods from town to town trying not to get mugged by monsters and theives

and when they make it they sell the goods and make a profit.