Baseball is played between two teams of nine or ten players each (unlimited substitutions are allowed from a total roster of 26 players, but a player once removed may not return). At each point, one team is said to be "on offense" (equivalently "up to bat", "batting", etc.) and one is said to be "on defense" (equivalently "fielding", "pitching", etc.). The team on offense remains so until three of its players have been declared out; the meaning of this term will be explained later. At that point, the other team takes a turn (after a short break), and so on, repeating in this way until each team has had nine chances on offense. Each alternation of both teams is called an "inning"; one team's turn is called (not surprisingly) a "half-inning".

The field of play is comprised of a partially-dirt area called the infield, and a totally grassy area extending out from it, called the outfield. Within the infield are four spots designated by a white mat, arranged in a 45-degree-rotated square, called bases. The names of these bases are, starting from the one on the bottom and proceeding counter-clockwise: home plate, first base, second base, and third base. Roughly (but not exactly) at the center of the square defined at the bases is a raised mound of dirt called the pitcher's mound.

The field of play is bounded on the left and right by two lines that intersect at home plate. It is bounded on the far edge by a roughly arc-shaped wall; two large yellow poles are set in the left and right edge, respectively, of this wall, to make it easy to tell at a glance which part of it is in fact in the field of play.

In baseball, points are idiosyncratically called runs; the object of the game is to have scored more runs than the opposing team at the end of nine innings.

A run is scored by a player from the team on offense starting near home plate, then touching all four bases in succession: first base, second base, third base, and home plate again, without being declared out. The conditions under which a player is allowed to enter the base paths, as well as under which he is declared "out", will be described below.

If a team is using ten active players, nine of them participate on offense, and nine participate on defense. The player who participates only on defense, but not on offense, is the pitcher; the one who participates on offense, but not on defense, is the designated hitter. A team may use only nine players, in which case the pitcher and designated hitter are the same person. Teams rarely choose to do this, since it is unusual for a player to be simultaneously gifted at pitching and hitting.

The nine players on a team who participate on offense are arranged in a batting order; this order may not be changed during the game (though, as discussed above, substitutions are possible). At the end of one player's turn at bat, the next player in the order takes his place in the designated batting area near home plate.

During a player's turn at bat, the team on defense is arranged in the field: the pitcher stands on the pitcher's mound, and the other eight defense participants are arranged throughout the infield and outfield, in positions that have become standard over time.

The pitcher then throws a series of baseballs towards the batter; each such act is called a pitch. If a pitch is positioned in such a way that the batter has a reasonable chance to hit it, it counts as a strike, regardless of whether the batter actually attempts to hit it or not. This "strike zone" depends largely on the umpire's judgment, but roughly corresponds to the pitch passing over home plate, and between the batter's knees and the midpoint of his torso. If the batter swings at a pitch, attempting to hit it, it counts as a strike, regardless of whether it passed through the strike zone or not. If the pitch does not pass through the strike zone, and the batter does not swing in an attempt to hit it, the pitch counts as a ball.

If three "strikes" are thrown without the batter successfully hitting the ball into the field of play, he is out. Otherwise, if four "balls" are thrown, he is allowed to proceed freely to first base (and is now a quarter of the way towards scoring a run). Thus, the pitcher is incentivized to either throw pitches that are as close as possible to the edge of the strike zone and thus difficult to hit while still being strikes, or to throw pitches that look deceptively like they will be strikes and then curve away at the last moment, enticing the batter to swing at ultimately unhittable balls. To achieve this affect, a variety of techniques are used, mostly involving spinning the baseball in various ways while throwing it. The batter is incentivized to only swing at pitches that he judges will count as strikes; quick reaction time and reflexes are therefore of utmost importance for skilled batters.

If the batter successfully hits the pitch, several scenarios are possible:

1. The ball lands to the left, to the right, or behind the field of play; that is, in foul territory. If fewer than two strikes have been recorded against the batter, this counts as a strike. Otherwise, it has no effect. In either case, the batter returns to home plate to continue his turn at bat.
2. A defensive player catches the ball in the air, before it touches the ground. In this case, the batter is immediately "out". It does not matter whether the ball would have landed in foul territory or not.
3. The ball hits the ground (in fair territory; i.e., not in foul territory). The batter then becomes a runner; he attempts to proceed at least to first base (continuing, if time permits, to further bases; second base, third base, or even home plate), while the defensive players collaborate to put him "out" before he can do so; the rules governing this phase of play are described more thoroughly in the "Baserunning" section below.
4. The ball leaves the field of play by going over the outfield wall (in between the two yellow posts described in the section "The field" above), without first touching the ground. This is treated essentially as though the ball landed in fair territory infinitely far away, and it would take the fielders infinitely long to attempt to retrieve it and put any of the runners out. Thus the batter (and any other runners that may be on any of the bases) are allowed to proceed safely to each of the bases in order, ultimately all scoring runs at home plate. This event is called a home run.

A few more unusual and rare scenarios have been omitted here.

If the batter becomes a runner and safely makes it to one of the bases (most usually first base, as the bases must be touched in order) without being put out, he is entitled to stay at that base safely while the next (and possibly subsequent) batters take their turns at bat, and in the future attempt to advance to a subsequent one.

Runners may be put out in one of several ways:

1. If at any time a runner not on any of the bases is touched by a defensive player who is holding the baseball in the hand he touches him with, that runner is out. (Exception: a runner who was just a batter is allowed to over-run first base for a few feet in a straight line, having become "safe" as soon as he touched the base. This exception does not apply to any of the other bases).

2. If a runner is forced to advance to the next base (which will be defined presently), any fielder may put him out by touching or stepping on that base while holding the baseball. Being "forced" to advance is defined recursively, as follows: a batter who has just batted and become a runner is forced to advance to first base. If any runner is forced to advance to a given base, and another runner occupies that base, he is forced to advance to the next base.

   Examples:

   • Runners occupy both first base and second base, and the batter hits the ball on the ground in play. The runner who was just a batter is "forced" to advance to first base. The runner who was on first base is "forced" to advance to second base. The runner who was just on second base is "forced" to advance to third base. If a defensive player, while holding the baseball, steps on second base, the runner on first base is out. Thus, the player who was previously on second base is no longer forced to advance to third base, as there is now nobody who is forced to advance to second base!
   • A runner occupies second base, and no other base is occupied. The runner who was just a batter is forced to advance to first base, as always. However, since nobody is forced to advance to second base, the runner at second base is not forced to do anything; he may choose to stay at second base, or advance to third base or even home plate (at risk of being tagged out on the way), at his discretion.

3. If the ball is caught in the air (putting the batter out), all the runners must touch the last base they had occupied at the time the ball was hit; if they have advanced beyond it, they are therefore forced to go backwards. As with all forced base movement, they may be put out merely by a fielder stepping on base while holding the ball; it is not necessary to actually tag the running player.

   Example: One out has been recorded in the half-inning. A runner is on second base. The batter hits a ball deep to the outfield; the runner judges that it is impossible for a fielder to catch in the air, and starts running towards third base. However, a fielder makes an amazing run and diving catch, putting the batter out. The runner must now return to second base. It does not matter if he has already touched third base or even home plate. If the fielder throws the ball to another fielder standing at second base, who touches it before the runner successfully returns, that runner is out, and the half-inning is over (as there have been three outs: there was one when the play started, and both the batter and runner are also now out).

It is important to note that runners may advance or be put out at any time while play is live, not merely after a batter hits the ball. That is, even if the pitcher is still holding the ball, a batter may attempt to advance, and the pitcher may throw the ball to any of the other fielders to attempt to put him out. A runner who successfully advances to the next base while the pitcher was holding the ball is said to have stolen a base. A runner who is tagged out while attempting to do so is said to have been caught stealing.