These are special classes of crater. Walled plains are between 60 and 300 km in diameter, and typically will have large, smooth floors with no central peak. The walls around their perimeters are low or absent and frequently appear polygonal. They give the (misleading) impression of having been formed by subsidence rather than impact. Clavius, Posidonius and Ptolemaeus are good examples.
Ring mountains are smaller, between 20 and 100 km across, and have circular walls with sharp, well-defined summits and, generally, large central peaks. Copernicus, Theophilus and Tycho are typical.