Corns are hyperkeratoses of the skin.
This a a thickening of the surface layer of the skin in response to pressure. Corns usually form on the toes, where the bone is prominent and presses the skin against the shoe, ground, or other bones. As corn become thick the tissues under the corn are subject to increased irritation. There may be a deep seated nucleation , this is like a core where the corn is thickest and most painful. As corns become inflamed, there is pain and sometimes swelling and redness.