A batter in baseball is a player in the batter's box attempting to hit the ball thrown by a pitcher. He is at home plate to attempt to reach base and eventually score by advancing to first base, second base, third base, and home plate.
Every member of the offense steps up to bat in turn. When at bat, the hitter must successfully make contact with the ball before accumulating three strikes. Upon the third strike, the hitter is "out", and the next player in the lineup is called up to bat.
When the hitter steps up to bat, the pitcher of the opposing team on defense will throw the ball to the catcher, positioned behind the hitter. A strike is called by the umpire if the hitter fails to bat a ball that is traveling within the strike zone. A strike is also called if the hitter who has less than two strikes bats a foul ball. However, a strike will not be called in the case where a hitter with two strikes bats a foul ball. (Note that a foul ball refers to when the hitter successfully bats the ball, but it falls outside of fair territory.)
In addition, a "ball" refers to the instance where the pitcher throws the ball outside the strike zone and the hitter makes no attempt to bat it. When four balls have been called and the hitter has not yet accumulated three strikes, the hitter will be given a "walk," or a free pass to first base. All other baserunners present will also be allowed to advance onto the next base.