Corns are hard cone-shaped bumps of skin commonly found on the upper surface of the smaller toes, particularly over a joint. Calluses are somewhat rounded flat thickenings of the skin located on the under-surface of the foot.
Corns and calluses are usually caused by friction and pressure, particularly from tight or ill-fitting shoes. Hammer toe and other toe deformities are often responsible for the development of corns on the top of or at the tip of the toes. Calluses often develop under the ball of the foot because of faulty foot positioning and poor weight distribution. Symptoms include a generalized burning sensation or (at times) severe pain in a specific area. People who have diabetes and a diminished sensation to light touch and protective sensation are at increased risk of developing ulcers and an infection at the site of the callus or corn if left untreated.
Treatment usually requires removal through scraping with a scalpel. After this procedure, padding of various sorts (for example, felt or moleskin) may be applied to remove pressure from the healing area. Devices that change the position or range of movement of the foot and relieve pressure on affected joints (orthoses) or other inserts that have padding and metatarsal support pads can help to reduce pressure caused by callus build-up under the balls of the feet. Dells, which are holes cut through part of the footwear beneath the area that is painful, can also help reduce pressure and pain.
If the blood supply to the affected area is poor, surgical removal of the dead tissue may not be possible. In this case, special shoes that reduce pressure over the affected area may be necessary.
Resource: Merck Manual Home Edition