Baseball is played between two teams of nine players each on a baseball field, usually under the authority of one or more officials, called umpires. There are usually four umpires in major league games; up to six (and as few as one) may officiate depending on the league and the importance of the game. There are four bases. Numbered counter-clockwise, first, second and third bases are cushions shaped as 15-inch (38 cm) squares which are raised above the ground; together with home plate, the fourth base, they form a square with sides of 90 feet (27.4 meters) called the diamond. Home base is a pentagonal rubber slab known as home plate. The field is divided into two main sections: the infield contains the four bases, and beyond two adjacent sides of the diamond there is an outfield. The other two sides of the diamond form the start of the foul lines, which extend straight, and form the boundary in the outfield as well.

The game is played in nine innings in which each team gets one turn to bat and try to score runs while the other pitches and defends in the field. In baseball, the defense always has the ball -- a fact that differentiates it from most other team sports. The teams switch every time the defending team gets three players of the batting team out. The winner is the team with the most runs after nine innings. In the case of a tie, additional innings are played until one team comes out ahead. At the start of the game, all nine players of the home team play the field, while players on the visiting team come to bat one at a time. A batter follows through after swinging at a pitched ball.

The basic contest is always between the pitcher for the fielding team, and a batter. The pitcher throws—pitches—the ball towards home plate, where the catcher for the fielding team waits to receive it. The batter stands in one of the batter's boxes and tries to hit the ball with a bat. The catcher's job is to catch any ball that the batter misses or does not swing at, and, most importantly, to "call" the game by a series of hand signals to the pitcher what pitch to throw and where. If the pitcher disagrees with the call, he will "shake off" the catcher by shaking his head no; he accepts the sign by nodding. The catcher's role becomes more crucial depending on how the game is going, and how the pitcher responds to a given situation. Each pitch begins a new play, which might consist of nothing more than the pitch itself.

Each half-inning, the goal of the defending team is to get three members of the other team out. A player who is out must temporarily leave the field and await for his turn to bat to arrive again, thus he cannot produce any more offense until then. There are many ways to get batters and baserunners out; some of the most common are catching a batted ball in the air, tag outs, force outs, and strikeouts (also known as Ks). After the fielding team has put out three of the batting team's players, the half-inning is over and the team in the field and the team at bat switch places. A complete inning consists of each opposing side having a chance to score.

The goal of the team at bat is to score runs; a player may do so only by batting, then becoming a base runner, touching all the bases in order (via one or more plays), and finally touching home plate. To that end, the goal of each batter is to enable baserunners to score or become a baserunner himself. The batter attempts to hit the ball into fair territory—between the foul lines—in such a way that the defending players cannot get him or the baserunners out. In general, the pitcher attempts to prevent this by pitching the ball in such a way that the batter cannot hit it cleanly.

A baserunner who successfully touches home plate after touching all previous bases in order scores a run. In an enclosed field, a fair ball hit over the fence on the fly is normally an automatic home run, which entitles the batter and all runners to touch all the bases and score.