A spur is a sharp horn-like protrusion that can grow on the legs of chickens, and is used for fighting and self-defense. Although all chickens have the potential to grow spurs, large spurs are most common on roosters. Hens can and sometimes do have large spurs, but generally do not. Spurs on a rooster start out very small and may take a year or more to become sharp.
If a spur is cut off or broken, it can bleed profusely and even kill the chicken from blood loss if the bleeding is not stopped. Although some people prefer to remove the spurs from their roosters, great care has to be taken, and the right methods used to avoid blood loss.