Repeated and intermittent pressure on the soles of the feet or the hands can lead to the formation of thickened layers of skin over an extended period of time. A callus is a yellowish layer of hard skin and a corn is a cone-shaped part of that same hard skin pushing into the skin and causing pain. If left untreated corns can ulcerate and even become infected. On the contrary calluses are protective layers of hardened skin that form over the points of contact as protective layers.