A callus is a thickening or hardening of the skin. They form as a result of friction or pressure against the skin. In general, calluses are normal and natural. They are part of the body’s defense system. For example, Since they have never walked, infants have very tender feet. They have no calluses. But adults often have hard, even horny calluses. People that are frequently barefoot may develop calluses on their feet that are as thick and hard as leather.