Between the late 1800s and the early 1960s, the debate over the origin of the Moon's craters seesawed between highly explosive volcanoes and impacts. You can follow up this debate here . The entire Moon is bone-dry, but these terms are still used to describe the basic divisions. The maria tend to be lowlands, the terra highlands. The terrae are very rough and heavily cratered.
Craters are still abundant on the maria, but there are far fewer than on the terrae.
Mercury Mercury looks much like the Moon. There are some smooth areas that may be lava plains, but it's hard to tell. Mars Mars has a greater diversity of surface features than Mercury or the Moon. Impact craters are still abundant, but volcanoes and lava plains are more evident. There are also huge scars formed by crustal extension and erosion.