A corn is formed by the skin when it is under pressure. The skin firstly thickens up to try to protect itself against the increased pressure on it. Once the skin thickens this actually creates more pressure on the area and the skin thickens up some more. This process carries on until the thickened skin gets compressed down into a corn. So a corn is really compressed hard skin. They can be very painful as they press on the deep layers of the skin and nerves.