A wart is a viral infection of the surface layers of the skin. The incubation period varies from a few weeks to several months. Warts can be spread by direct or indirect contact with a wart to damaged skin.
A corn is a small, hardened area of skin which often looks yellow compared to the surrounding skin. It is typically round or corn-shaped, pointing down into the skin. Corns most often form on the feet and sometimes on the hands and are caused by constant or repeated friction or pressure.
A callus is rough, thickened skin spread over a wide area. Like a corn, it caused by constant or repeated friction or pressure, but unlike corns, calluses are flat and have normal skin markings.