Fitzgerald uses a number of things to characterize the people in The Great Gatsby including their clothes and their shape and their backgrounds but one of the most powerful ways that he differentiates them is by describing the way they speak and in crafting their conversations.
He describes Daisy as having "the voice of money," and it has a great power anyone who listens to it. It serves to emphasize her desire for a husband that can provide a certain type of life for her. She also uses her voice to continue to flirt with him and maintain his interest.
Tom Buchanon, on the other hand, talks about the "other races" that are going to take over the world and he speaks sharply and in hard ways that serve to emphasize his sense of being aloof and better and more important than the people around him. He speaks in commanding tones and doesn't brook disagreement, a direct contrast to the almost hesitant way Gatsby speaks until the direct confrontation with Tom about Daisy.