Lyot stands out on the flat plains of Vastitas Borealis, which is generally flat and smooth with few large craters. Lyot is the deepest point in Mars's northern hemisphere. (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.