A Mars rover is an automated motor vehicle which propels itself across the surface of the planet Mars after landing.
Rovers have several advantages over stationary landers: they examine more territory, they can be directed to interesting features, they can place themselves in sunny positions to weather winter months and they can advance the knowledge of how to perform very remote robotic vehicle control.
There have been three successful Mars rovers, all of which were robotically operated. The Jet Propulsion Laboratory managed the Mars Pathfinder mission with its Sojourner rover and currently manages the Mars Exploration Rover Mission with its two rovers, Spirit and Opportunity, for NASA’s Office of Space Science, Washington, DC. (There have also been two successful non-Martian robotic rovers: in the 1970s the USSR sent two Lunokhod rovers to the Moon.)