A corn is a condensed mass of hard skin that is shaped in a cone shape with a tip. They can be very painful and can feel like a stone in your shoe. Corns can be removed quite easily and without any pain or discomfort but they may return if there is constant repetitive friction or pressure on one particular point of the foot.
Callus is usually caused by excessive pressure or friction over an area of the foot eg. over a bony area or on the heels.