From BR Bullpen
The pitching rubber or pitcher's plate is a flat rectangular slab made of whitened hard rubber (or sometimes wood) on top of the pitcher's mound which the pitcher must touch while beginning his motion to throw. Most pitchers work from the center of the rubber, using it to push off with their back foot to obtain additional velocity on their pitches. A few pitchers work from the side of the rubber, however, maintaining contact with the side of their foot while starting their motion; this allows them to throw a pitch coming from an angle which is uncomfortable for the batter.
The rubber's dimension is 24 inches by 6 inches, according to section 1.07 of the rules. The pitcher's rubber is set so that its front edge is exactly 60 feet 6 inches from the rear point of home plate, and is elevated 10 inches above the rest of the playing field. The area of the mound around the pitching rubber is flat.
The rubber comes into play often in defining what is a balk.