Calluses are hardened areas formed on the skin - mostly on your hands and feet - due to immense pressure or friction that they have gone through. They are formed when continued force is placed on the area and is common for CrossFitters or those who do lots of walking with poorly-fitted shoes. They are not necessarily harmful but can be a bother each time you look at them, or they squeeze against your toe when wearing certain shoes. However, if they are not cared for, they can become infected and painful each time they grazed against a hard area.