A callus is simply a buildup of skin on area of pressure.  If there is abony prominence on one toe that is rubbing up against the toe next to it, your body tries to "protect" itself by producing a barrier or skin.  This skin buildup can become so thick that it appears as a callus.  Do not try to treat this yourself; instead make an appointment with one of our doctors to find out why it is occuring, and to find ways to prevent it.