Callus is caused by friction which our foot is constantly subjected to in our daily walking. As we walk our skin rubs on shoes or socks, some parts of the foot are subject to more friction than other parts and as a result the body tries to protect itself by forming thick layers of skin. Callus is often good for example a guitar player will build up layers of callus on her finger tips to protect them from the hard strings but if callus builds up underneath the foot then it causes pain when we stand on it. As the callus develops further then further problems develop as the body tries to protect itself and the pain increases.
The callus is simply removed layer at a time until the foot is smooth and the area blends in with the good, healthy skin that surrounds it.
The same forces that cause callus build up are the same forces that cause blisters. If the friction and pressure is often and hard enough then layers of skin can be removed which leave walking very painful. Only when the pressure is not too hard and frequency is right will callus start to build up and begin to protect or annoy us.
Friction and pressure are the cause of most callus but there are other causes too. Some toxins can create callus as can diseases such as syphilis which can cause thickening of the skin on the bottom of the foot and spots of callus to develop.
Good fitting shoes may help to prevent the build up of callus and some shoes for example sandals and flip flops may increase the build up of some callus.