Callus appears on the foot as a yellowish area of thickened, tough skin. A corn is made of similar hard skin, but has formed into an inverted cone that pushes deeper into tissue. Both of these problems are associated with excess pressure and/or friction on the foot, and can become very painful.
The cause of this excessive pressure may include over exercising, poor fitting or poor quality footwear, high arches or flat feet, being overweight, old age, and pregnancy.