Calluses form on the areas of skin, where there is a close contact or pressure. They are thickened, hornificated dead cells of epithelium. They often occur on feet, due to wearing of uncomfortable shoes and on hands, when a person does heavy work or during physical activity. There are two types of Calluses, dry and wet( when there is a blister filled with fluid). Calluses can be painful. 
How to prevent calluses? 
* Wear comfortable shoes
* Put on gloves while your are doing some heavy work (hammer, saw) or at a gym
* Calluses can heal on their own, but sometimes it’s very hard to get rid of them. 
* Treatment includes medicine, that exfoliates the skin and mechanical removal of keratinized areas.