The offense attempts to score runs by hitting a ball thrown by the pitcher with a bat and moving counter-clockwise around a series of four bases: first, second, third, and home plate. A run is scored when a player advances around the bases and returns to home plate.
Players on the batting team take turns hitting against the pitcher of the fielding team, which tries to prevent runs by getting hitters out. A player on the batting team who reaches a base safely can later attempt to advance to bases during teammates’ turns batting, such as on a hit. The teams switch between batting and fielding whenever the fielding team records three outs. One turn batting for both teams, beginning with the visiting team, constitutes an inning. A game comprises nine innings, and the team with the greater number of runs at the end of the game wins.
Within the Official Rules (last ammended in 2014) of Major League Baseball, Section 1.0, Objectives of the Game, there is a rule for the ball.
1.09 The ball shall be a sphere formed by yarn wound around a small core of cork, rubber or similar material, covered with two strips of white horsehide or cowhide, tightly stitched together. It shall weigh not less than five nor more than 5¼ ounces avoirdupois and measure not less than nine nor more than 9¼ inches in circumference.
Major League Baseball Rule 1.09 – http://mlb.mlb.com/mlb/downloads/y2014/official_baseball_rules.pdf
Photo – www.androidguys.com/2011/03/16/ready-opening-day-10-baseball-apps-android/