Most commonly, callus is a small area of skin (usually on the foot) which has become relatively thick and hard, from rubbing and/or pressure. Calluses may lead to other problems such as serious infection. Shoes that fit well can keep calluses from forming.
Players of stringed instruments such as the guitar will develop calluses on their fingers if they play frequently enough. They are a good thing in this case, because without them, playing the instruments for longer than a few minutes (or for someone who just started, a few seconds!) can be extremely painful. Sometimes a blister is produced before a callus.
See also: fibrocartilage callus