Corns are caused by pressure or friction over bony areas, such as a joint, and they have a central core which may cause pain if it presses on a nerve.
Callus (or callosity) is an extended area of thickened, hard skin on the soles of the feet. It is usually symptomatic of an underlying problem such as a bony deformity, a particular style of walking or inappropriate footwear. Some people have a natural tendency to form callus because of their skin type. Elderly people have less fatty tissue in their skin and this can lead to callus forming on the ball of the foot.