Heimdal is a recent impact crater on the planet Mars. The crater is approximately 20 kilometers from the landing site of the Phoenix lander. The landing site area is believed to be blanketed by ejecta excavated by the impact that created Heimdal, approximately 600 million 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.