Orbiting Mars are two moons, Phobos and Deimos. This is a picture of the largest of the two moons, Phobos, which comes from the Greek word for fear. Phobos is only 17 miles across, and its orbit around Mars is only 8 hours long, zipping across the sky from the western horizon to the eastern horizon.
Fun Fact: Phobos is doomed. In 100 million years Phobos will either crash into Mars or break up as it's dragged down by the planet's gravity. The current distance between Phobos and the surface of Mars is about 3,600 miles, compared to 250,000 miles between the surface of the Earth and its Moon.
Photo source: NASA