A callus is a thickened layer of keratin the skin develops to protect itself in areas subject to constant pressure or friction. The sole of the foot, under the toes, the heel and the hands are the main areas where they're seen. They're firm, yellowish or greyish in colour, and feel raised and hard compared with the surrounding skin. They're usually painless. They often form after wearing unsuitable shoes, doing heavy manual work, or athletic activities.
A callus is a protective adaptation of the skin – in other words, it's actually the body's own defence mechanism. There is thickening in the corneal layer (stratum corneum), the outermost layer of the skin. In terms of treatment, reducing pressure and friction (using suitable shoes, insoles, gloves) and regularly applying moisturisers and urea-containing creams is sufficient in many cases. In more severe and widespread cases, it's appropriate to consult a dermatologist for a differential diagnosis and further treatment.