Copernicus is a large crater on Mars, with a diameter close to 300 km. It is located south of the planet's equator in the heavily cratered highlands of Terra Sirenum in the Phaethontis quadrangle at 48.8°S and 191.2°E. The impact that formed Copernicus likely occurred more than 3 billion years ago. The crater contains smaller craters within its basin and is particularly notable for gully formations that are presumed to be indicative of past liquid water flows. (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.