Wikipedia: Baseball

Baseball is a team sport, popular in the Americas, East Asia and Australia. In its usual form, the game is between two teams of nine players on a playing field consisting of 4 bases, arranged in a diagonal square ("the diamond") and a large outfield (see /Fielding positions for a typical layout).

Play of the Game

As the game starts, the home team takes the field, while the visitors come to bat. After making three outs, the visitors take the field and the home team bats.

The basic contest is always between the pitcher, who tries to throw the ball so it cannot be cleanly hit, and the batter, who tries to hit the pitched ball with a rounded bat. If the batter hits a "fair ball" into the field of play, the hitter runs to first base and any of his teammates who are already "on the bases" may attempt to advance to another base. If a baserunner is already on first base, they must try to advance or the batter will be out; no two offensive players may ever stay on the same base. The batting team scores a run by advancing a player all the way around the diamond.

If the ball is caught before it bounces, or the ball is fielded and thrown to a base before a runner arrives there, the player is out, and must return to his team's dugout. There is also an imaginary area above "home plate" (where the batter stands) between the batters knees and chest called the "strike zone". Any pitch which passes through this area is a "strike", as is any pitch at which the batter swings and misses. If a batter records 3 strikes before putting the ball in play, he is out. (An exception is if the third strike evades the catcher but this rarely occurs.) Any pitch which is not a strike is called a Ball. A batter who receives 4 balls from a pitcher may walk to first base and cannot be tagged out. This is called a "walk." A batter may also move to first base if he is struck by a pitched ball, unless he puts himself in the path of the pitch and makes no attempt to avoid being struck.

After 3 outs (a "half-inning") the roles of the fielding and hitting sides are reversed. Usually, 9 innings are played. The aim of the game is to score more runs than the opposing team. If both teams have scored the same number of runs at the end of 9 innings, an extra inning is added to the game. If the score remains tied, another inning is added. This process repeats until the score is no longer tied at the end of an inning. Thus, the team which hits in the second (or "bottom") half of the inning always has a chance to respond to a run scored by the team batting in the first (or "top") half. As there are tactical advantages to this, the home team is always granted the right to bat in the bottom half of the inning. Baseball games end with tie scores only because weather or lighting conditions have made it impossible to continue play. In the Major Leagues, teams usually continue games at a later date rather than allow a game to end in a tie.

Pitching

Effective pitching is vitally important in baseball. A pitcher who starts games should be able to pitch for 6 or 7 seven innings before being replaced by specialist relief pitchers, who finish the game off. For a starter to pitch all 9 innings (a "Complete Game") is a personal achievement, though this was not always so -- the average number of innings pitched has been declining slowly in the professional leagues almost since their inception, and 9 innings was once the norm. Pitching is also physically demanding: a modern-day starting pitcher can usually throw 100-110 pitches with no ill effects, but throwing many more reduces effectiveness, and sometimes serious and permament arm injury. In a major league season then, a club usually keeps a cadre of 5 starting pitchers (known as the "starting rotation") to start games, giving pitchers at least 3 or (preferably) 4 or 5 days rest between starts. 5 to 7 more pitchers are employed to pitch the innings not handled by the starting rotation.

Types of Pitch

In order to prevent the batter from hitting the ball well, a good pitcher should be able throw a variety of different pitches, which will usually be a subset of the following basic types.

Fastball: The fastball is the pitch that most pitchers throw most of the time. Some "power" pitchers can throw it 95-100 mph, and rely on this speed to prevent the ball being hit. Others throw more slowly but put movement on the ball or throw it on the outside of the plate where the batter cannot easily reach it.
Curve ball: The curve ball is thrown with a hand motion that induces extra rotation on the ball causing it to "break," to fly in a more exaggerated curve than would be expected. The pitch is slower than a fastball, and this difference in velocity also tends to disrupt the hitter's timing. Good curve balls often seem to drop sharply as they reach the plate, making the batter swing above it; but a curve ball which fails to break (a "hanging curve") will be easy meat for a good hitter. A Screwball is similar to a curveball, but thrown from the back of the hand in order to impart opposite rotation and opposite movement.
Slider: A slider is half-way between a curve ball and a fastball, with less break but more speed than the curve. It will tend to drop less and move toward or away from the batter more than a curve. The extra speed can fool the hitter into thinking it's a fastball, until too late. Some pitchers also use a cut fastball (or cutter) which is one step closer than the slider to the fastball on the spectrum between fastballs and curves.
Change Up: A change up is the traditional off-speed pitch (i.e. slower than the fastball), which otherwise resembles a fastball. It is thrown with the same arm action as a fastball; the speed difference is due to a different grip. This (hopefully) causes the hitter to be fooled and swing before the ball arrives. A change up also tends to break slightly in the same direction as a screwball due to the way it is commonly released, this makes it an effective pitch away from the plate.
Knuckleball: Thrown slowly and with a minimum of rotation, the knuckleball (actually thrown off the tips of the fingers) relies on chaotic airflow over the stitched seams of the baseball to produce an erratic, unpredictable motion. This makes it hard to hit, hard to catch and hard to aim, and it is consequently not a favorite with catchers and managers.
Split-Fingered Fastball / Forkball:Held between the first two fingers, thrown hard and with a strong downward motion. This pitch tends to tumble downwards and can break in either direction, depending on the release. It can be thrown as hard as 90 mph, so it can look like a fastball until it breaks near the plate. Most effective when thrown in the lower part of the strike zone.

The pitcher will try to make the batter miss the ball entirely ("go out on strikes") or hit it so that it can be handled by one of the fielders. This generally involves throwing the ball in a way, or to a location, that the batter is not expecting, causing him to hit it weakly or not at all. Since it is very difficult to do this, the batter will often be able to hit the ball strongly. Good fielders may have some idea of where the pitcher is likely to throw the ball, and therefore where the hitter is likely to hit it (an "outside" pitch will generally be hit to the side of the field that the batter faces, for instance), and may be prepared to field the ball there if the batter hits it well.

The batter tries to hit the ball in such a way that it cannot be cleanly handled by a fielder; a good hitters can "direct" a batted ball surprisingly accurately to some area of the playing field. Since the only certain way to prevent the ball being fielded is to hit it beyond the bounds of the playing field, "home runs" have become increasingly popular with hitters and managers.

Equipment

Bat: A solid wooden or hollow aluminum bat.
Ball: A cork sphere, tightly wound with layers of yarn or string and covered with a stitched leather coat.
Mitt: Leather glove worn by players in the field. Long fingers and a webbed "pocket" between the thumb and first finger allow the fielder to catch the ball more easily.
Catcher's Mitt: Leather glove worn by Catchers. Generally larger and better-padded than the standard fielder's mitt.
Batting glove: Glove often worn on one or both hand(s) by the batter. Offers additional grip on the bat.

Clothing

Hat: 'Baseball cap' worn by all players
Batting helmet: Protective helmet worn by batter to protect the head and the ear facing the pitcher from the ball.
Catcher's helmet: Protective helmet with face guard worn by the catcher.
Baseball Uniform: Shirt and pants worn by all players. Each team generally has a unique pattern of colors and designs.
Athletic supporter and cup: Worn by Catcher, and often by all players. Protects the male genitals from injury. 'Jockstrap', 'jock' or 'cup supporter'.
Sliding shorts: Padded support shorts sometimes worn to protect the thighs when the player slides into the bases.
Spikes: Shoes with spikes to provide additional traction. Historically used by sliding baserunners to intimidate fielders at the bag.