Baserunning

Once a batter becomes a runner and reaches first base safely, he is said to be "on" that base until he attempts to advance to the next base, until he is put out, or until the half-inning ends. When comparing two or more runners on the basepaths, the runner farther along is called a lead runner or a preceding runner; the other runner is called a trailing runner or a following runner. Runners on second or third base are considered to be in scoring position since ordinary hits, even singles, will often allow them to score.
A runner legally touching a base is "safe"—he may not be put out. Runners may attempt to advance from base to base at any time (except when the ball is dead), but must attempt to advance when forced--when all previous bases are occupied and the batter becomes a runner. When a ball is hit in the air, a fly ball, and caught by the defending team, runners must return and touch the base they occupied at the time of the pitch—called tagging up—after the ball is first touched. Once they do this, they may attempt to advance at their own risk.
Only one runner may occupy a base at a time; if two runners are touching a base at once, the trailing runner is in jeopardy and will be out if tagged, unless he was forced--in which case the lead runner is out when tagged for failing to reach his force base. Either such occurrence is very rare. Thus, after a play, at most three runners may be on the basepaths, one on each base--first, second, and third. When three runners are on base, this is called bases loaded.

Pick-off attempt on runner (in red) at first base
Baserunners may attempt to advance, or steal a base, while the pitcher is throwing a pitch. The pitcher, in lieu of delivering the pitch, may try to prevent this by throwing the ball to one of the infielders in order to tag the runner; if successful, it is called a pick-off. If the runner attempts to steal the next base but is tagged out before reaching it safely, he is caught stealing. An illegal attempt by the pitcher to deceive a runner, among other pitching violations, is called a balk, allowing the runners to advance one base without risk of being put out.
Another fundamental tenet of the rules of baseball is that a runner originally ruled out can subsequently be ruled safe, but once a runner is ruled safe he cannot be called out on the same play. A runner initially called out can be subsequently ruled safe if the fielder putting the runner out drops the ball (on either a tag or force play), pulls his foot off the base (in the case of a force play), or otherwise illegally obstructs a runner from reaching a base that he otherwise would have reached safely.

Batting and base running strategy
The goal of each batter is to become a base runner himself (by a base hit, a base on balls, being hit by the pitch, a fielding error, or fielder's choice) or to help move other base runners along (by another base hit, a sacrifice bunt, sacrifice fly, or hit and run).
Batters attempt to "read" pitchers through pre-game preparation by studying the tendencies of pitchers and by talking to other batters that previously faced the pitcher. While batting, batters attempt to "read" pitches by looking for clues that the pitcher or catcher reveal. These clues (also referred to as "tipping pitches") include movements of the pitcher's arms, shoulders, body, etc, or the positioning of the catcher's feet and glove. Batters can attempt to "read" the spin of a ball early in the pitch to anticipate its trajectory. Batters also remain keenly aware of the count during their at bat. The count is considered to be in the batter's favor when there are more balls than strikes (e.g.two balls and no strikes). This puts pressure on the pitcher to throw a strike to avoid a walk so the batter is more likely to get an easier pitch to hit and can look for a particular pitch in a particular zone or take a riskier or bigger swing. The count is considered to be in the pitcher's favor when there are fewer balls than strikes (e.g. no balls and two strikes). This gives the pitcher more freedom to try enticing the batter to swing at a pitch outside the strike zone or throwing a pitch that is harder to control (e.g. a curve, slider or splitter), but that is also harder to hit. Thus the batter will take a more protective swing. A major strategy in batting at competitive levels of baseball is patient hitting. An example of patient hitting is when a batter has a zero strike count the batter will almost always look for his perfect pitch. One strike hitting is very similar to no strike hitting and the batter usually is still looking for a good pitch to hit. Two strike hitting, the strategy is changed where the batter will protect the plate by fouling off pitches until the batter is able to find a pitch to hit. This style of hitting is known as patience at the plate. This style of hitting allows the hitter to look for a good pitch to hit and make the pitcher throw more pitches so that the pitcher will tire out faster. This is critical if the batting team is facing a very skilled pitcher who if allowed to will take over the game with his ability to get batters to do what he wants them to do with the pitches that he makes.

A third base coach (left) signals from the coach's box for the runner to slide head first into 3rd base
In general, base running is a tactical part of the game requiring good judgment by runners (and their coaches) to assess the risk in attempting to advance. During tag plays, a good slide can affect the outcome of the play. Managers will sometimes simultaneously send a runner and require the batter to swing (a hit-and-run play) in an attempt to advance runners. On a hit-and-run play the batter will often try to hit to the opposite field (the opposite of the natural tendency for the right-handed hitter to pull the ball to left field and vice versa). Hitting to the opposite field will likely find an opening in the infield vacated by the fielder covering second base. This is because coverage of second base against a steal is best achieved by whichever fielder is closer to second base, the shortstop or the second baseman; and such positioning is aimed at defending against the natural tendency of the hitter.
A batter can also attempt to move a baserunner forward by "sacrificing" his at-bat. This can be done by bunting the ball, hitting a fly ball far enough in the air that a baserunner can advance after the catch, or simply making contact with the ball on a hit-and-run play.
During the course of play many offensive and defensive players run close to each other, and during tag plays, the defensive player must touch the offensive player. Although baseball is considered a non-contact sport, a runner may be allowed to make potentially dangerous contact with a fielder as part of an attempt to reach base, unless that fielder is fielding a batted ball. (Noted exceptions to the dangerous contact rule are found throughout amateur competitions, including youth leagues, high school, and college baseball.) A good slide is often more advantageous than such contact, and "malicious" contact by runners is typically prohibited as offensive interference. The most common occurrence of contact of this nature is at home plate between the runner and the catcher, as the catcher is well padded and locked into position that completely blocks home plate from the runner, and the runner will often try to knock the ball out of the catcher's hand by running him over. Since the catcher is seen (symbolically and literally) as the last line of defense, it seems natural that the more physical play happens here.

Innings and determining a winner
An inning consists of each team having one turn in the field and one turn to hit, with the visiting team batting before the home team. A standard game lasts nine innings, although some leagues (such as high school baseball and Little League) play fewer. The team with the most runs at the end of the game wins. If the home team is ahead after eight-and-a-half innings have been played, it is declared the winner, and the last half-inning is not played. If the home team is trailing or tied in the ninth inning or beyond and they score to take the lead, the game ends as soon as the winning run touches home plate; however, if the last batter hits a home run to win the game, he and any runners on base are all permitted to score.
If both teams have scored the same number of runs at the end of a regular-length game, a tie is avoided by the addition of extra innings. As many innings as necessary are played until one team has the lead at the end of an inning. Thus, the home team always has a chance to respond if the visiting team scores in the top half of the inning; this gives the home team a small tactical advantage. In theory, a baseball game could go on forever; in practice, however, they eventually end. In addition to that rule, a game might theoretically end if both the home and away team were to run out of players to substitute (see Substitutions, below). In Major League Baseball, the longest game played was a 26-inning affair between the Brooklyn Robins and Boston Braves on May 1, 1920. The game, called on account of darkness, ended in a 1-1 tie. Two minor-league teams, the Pawtucket Red Sox and Rochester Red Wings, played a 33-inning game in 1981.
In Major League Baseball, games end with tie scores only in rare cases when conditions make it impossible to continue play. A tie game does not count as an official game in the standings unless it is finished later or replayed; however, individual player statistics from tie games are counted. Inclement weather may also shorten games, but at least five innings must be played for the game to be considered official; four-and-a-half innings are enough if the home team is ahead. Previously, curfews and the absence of adequate lighting caused more ties and shortened games. Also, with more modern playing surfaces better able to handle light rains, the process for calling or shortening a game due to weather has changed; it is more common than in the past to delay a game as much as 2 hours before a cancellation; also, a delay usually does not occur anymore until the rain is moderate-heavy and/or there is standing water on some part of the playing field.
In Japan's Nippon Professional Baseball, if the score remains tied after nine innings, up to three extra innings may be played before the game is called a tie. Some youth or amateur leagues will end a game early if one team is ahead by ten or more runs, a practice known as the "mercy rule" or "slaughter rule". Rarely, a game can also be won or lost by forfeit.
There is a short break between each half-inning during which the new defensive team takes the field and the pitcher warms up. Traditionally, the break between the top half and the bottom half of the seventh inning is known as the seventh-inning stretch. During the "stretch," fans in the United States often sing the chorus of "Take Me Out to the Ball Game;" since the September 11, 2001 attacks, "God Bless America" has often been added to it, especially at games in New York City.