The Mars Ascent Vehicle, commonly shortened to the MAV, is the space shuttle used by Ares astronauts to evacuate the surface of Mars.
The Ares III MAV was located in Acidalia Planitia close to The Hab. It was used on Sol 18 (Sol 6 in the book) by five of the six crew members when they had to evacuate early due to a dangerous dust storm encompassing the area. Astronaut Mark Watney was presumed dead on evacuation and the rest of the crew left without him. However, he survived but was injured by a piece of broken antenna.
The Ares IV MAV was located in Schiaparelli Crater and was placed there long before the date Ares IV mission was scheduled to take place. It was used by Ares III astronaut Mark Watney, who was accidentally abandoned on Mars by the rest of his crew.