Just to add to what Walter said, the twisted pair is what's called a balanced connection. That's to say, each wire carries the same signal, but they are in opposite phase (when one goes positive the other goes negative by the same amount). It's the voltage between the two wires which is the required signal. Because the wires are tightly twisted together, any interference is picked up equally in each wire, so it doesn't produce any voltage between the two wires to interfere with the signal.