Named after Robert Main - British astronomer (1808-1878).
The floor shows dark portions which are caused by pressurized carbon dioxide blowing dust in the atmosphere in the spring when the temperature goes up. Some of the dust is shaped into streaks if there is a wind. (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.