Klingon ambassador: Remember this well: there shall be no peace as long as Kirk lives!
Kirk: Mr. Spock, have you accounted for the variable mass of whales and water in your time re-entry program?
Spock: Mr. Scott cannot give me exact figures, Admiral, so... I will make a guess.
Kirk: A guess? You, Spock? That's extraordinary.
Spock: [to Dr. McCoy] I don't think he understands.
McCoy: No, Spock. He means that he feels safer about your guesses than most other people's facts.
Spock: Then you're saying... It is a compliment?
McCoy: It is.
Spock: Ah. Then, I will try to make the best guess I can.
McCoy: Please do.
Spock: Your use of language has altered since our arrival. It is currently laced with, shall we say, more colorful metaphors, "double dumb-ass on you" and so forth.
Kirk: Oh, you mean the profanity?
Spock: Yes.
Kirk: Well that's simply the way they talk here. Nobody pays any attention to you unless you swear every other word.
Spock: Forgive me doctor. I'm recieving a number of distress calls.
McCoy: I don't doubt it!
Answer: The United Federation of Planets uses the credit. Its a purely electronic form of money. Necessities and luxuries both are simple and cheap to produce with the Federation's advanced technology, and humanity has matured to the point that accumulating wealth is considered vulgar. Furthering the common good or the advancement of humanity is the real status symbol in the 23rd and 24th century. These conditions result in a society with very little need for money. Citizens are paid, but since the technology built into a place of business (or starship) or home supplies all basic needs for free, most people spend money only on exotic products that aren't commonly manufactured, like art or handmade foods.
Grumpy Scot