Who is Thrasymachus from Great Dialogues and what is their importance?

Thrasymachus is a participant in the discussion at Cephalus' house in "The Republic". He seems to be particularly averse to Socrates and his philosophical method, which is made clear by his rudeness and insulting behavior. While debating Socrates, he asks for payment before making his arguments (he never receives payment). It is believed that he is a member of a group of philosophers named the Sophists, a group that Socrates later condemns.