Earth orbits around the sun a bit more quickly than Mars. Most of the time, the apparent motion of Mars, as seen from Earth, has Mars slowly making steady progress across the background starfield. Over the past few months, however, Mars has appeared to be traveling backwards. This is because Earth caught up to, and passed the point of Mars orbit around the sun. The Astronomy Picture of the Day has an annotated version of this photo. It's an interesting astronomical phenomenon.
See Mars from Whidbey Island.