A callus is simply a thickening of the skin. It is the result of repeated pressure or contact. They are generally harmless, but if ignored calluses can enlarge and become painful.
The first step to removing a callus is to soak your feet in warm water to soften the callus. Using the coarse side of a callus rasp, scrape the callus gently. After you have scraped off of the majority of the callus, use the smooth side to finish. Wash feet with soap and water. You may have to repeat the process several times before calluses disappear.