A corn and a callus are virtually the same thing.  It is a spot on the foot that is made up of overgrown and thickened skin.  These are caused by bony abnormalities such as hammertoes, bunions and prominent bones on the ball of your foot.  If it is on the top of your foot, we call it a corn....if it's on the bottom we call it a callus.  However both are simply areas of thickened skin that can not be ignored.  If you have one, don't just do an 'at home pedicure' come in for a full evaluation.