Answer
If a listener moves toward a stationary sound source, and wave crests are heading toward her from the source, she will encounter these wave crests more frequently. The listener will hear a higher frequency.
On the other hand, if a listener moves away from a stationary sound source, and wave crests are heading toward her from the source, she will encounter these wave crests less frequently as they try to catch up to her. The listener will hear a lower frequency.
In summary, the Doppler effect is a change in perceived frequency when the wave source moves, when the receiver moves, or when both move.