Virrat is a Martian impact crater, approximately 54 kilometres in diameter. Several Virrat crater radii to the north are the craters Koga and Nhill. According to a surface age map of Mars based on US Geological Survey data, the area around Virrat is from the Noachian epoch, which places the area's age at 3.8 to 3.5 billion years ago. (wikipedia entry)
A crater is a circular depression likely created by an impact event. On Mars large craters are named after deceased scientists, writers and others who have contributed to the lore of Mars. Small creaters (less than 60km) are named after small towns and villages.