A callus (or callosity)'s an especially toughened area of skin which has become relatively thick and hard in response to repeated friction, pressure or other irritation. Rubbing that's too frequent or forceful will cause blisters rather than allow calluses to form. Since repeated contact's required, calluses are most often found on feet because of frequent walking. People who frequently play the guitar often also develop calluses on their fingertips due to the frequent friction on the strings.… (More on Callus)