Corns and calluses are both formed by a build-up of hard skin. Corns are usually smaller and can present as hard skin usually on the toes (although they may be on other areas of the feet). In contrast, a callus is a larger patch of hard skin, most commonly found on the sole of the foot.
In very basic terms, a corn or callus is caused when there is excess pressure rubbing on the skin, causing it to build up a thicker outer layer to protect the more sensitive layers.
Calluses and corns are actually your body’s way of protecting you from the high pressure, by building up the skin, which is why you’ll find most calluses and corns on high pressure points across your feet.