Substitutions

Each team is allowed to substitute for any player at any time the ball is dead. A batter who replaces another batter is referred to as a pinch hitter; similarly, a pinch runner may be used as a replacement for a baserunner. Any player who replaces another player between innings is known as a "defensive replacement". Any replacement is a permanent substitution; the replaced player may not return to the game.
It is common for a pitcher to pitch for several innings and then be removed in favor of a relief pitcher. Because pitching is a specialized skill, most pitchers are relatively poor hitters; it is common to substitute for a pitcher when he is due to bat. This pinch hitter is typically then replaced by a relief pitcher when the team returns to the field on defense, but more complicated substitutions are possible, most notably the double switch.
Many amateur leagues allow a starting player who was removed to return to the game in the same position in the batting order under a re-entry rule. Youth leagues often allow free and open substitution to encourage player participation.
The designated hitter (or DH) is a player whose sole purpose is to hit when it would normally be the pitcher's turn (or, if the pitcher is a good batter, another weaker batter). A few leagues, notably Major League Baseball's American League (which instituted the DH in 1973 to boost offensive output), allow designated hitters. This is not considered a substitution but rather a position, albeit a purely offensive one. A designated hitter does not play in the field on defense and may remain in the game regardless of changes in pitchers. The use of the designated hitter is opposed by many baseball traditionalists, but it is used today at most levels of baseball in the United States—high school, college, minor leagues—and internationally, including in the Olympics until Baseball's removal from the event. If the designated hitter is moved to a fielding position, the team loses the DH, and the fielder whose position was taken by the former DH is replaced by the pitcher, who assumes that player's position in the hitting lineup.

Rosters
The number of players on a Major League roster is dictated by the labor agreements worked out between players and management. According to the current rules, a team may have a maximum of 25 men on a roster from Opening Day until August 31. After that, teams may call up additional personnel, up to a maximum of 40 players on the active roster, with the exception of the postseason, where rosters are fixed at 25 men.

Other personnel
Any baseball game involves one or more umpires, who make rulings on the outcome of each play. At a minimum, one umpire will stand behind the catcher, to have a good view of the strike zone, and call each pitch a ball or a strike. Additional umpires may be stationed near the bases, thus making it easier to see plays in the field. In Major League Baseball, four umpires are used for each game, one near each base. In the all-star game and playoffs, six umpires are used: one at each base and two in the outfield along either foul line.

Baseball's unique style
Baseball is unique among American sports in several ways. This uniqueness is a large part of its longstanding appeal and strong association with the American psyche. The philosopher Morris Raphael Cohen described baseball as a national religion. Many Americans believe that baseball is the ultimate combination of skill, timing, athleticism, and strategy. In this, baseball is similar to its cousin game cricket: in many Commonwealth nations, cricket and the culture surrounding it hold a similar place and affection to baseball's role in American culture.

Time element
Basketball, ice hockey, American football, and soccer all use a clock, and games often end by a team with the lead killing the clock rather than competing directly against the opposing team. In contrast, baseball has no clock; a team cannot win without getting the last batter out and rallies are not constrained by time. Other sports popular on the professional level in the U.S. that do not have a time limit are tennis and golf, although these are individual as opposed to team sports.
In recent decades, observers have criticized professional baseball for the length of its games, with some justification as the time required to play a baseball game has increased steadily through the years. At the turn of the 20th century, games typically took an hour and a half to play. In the 1920s, they averaged just under two hours, which eventually ballooned to 2 hours and 38 minutes in 1960.[6] Though this average dipped to 2 hours 25 minutes in 1975, [7] by the turn of the 21st century, games had become so long that Major League Baseball's goal in 2004 was to get the average game down to 2 hour and 45 minutes, after coming close in 2003 at 2 hours and 46 minutes.[8]
The lengthening of games is attributed to longer breaks between half-innings for television commercials, increased offense, more pitching changes, and a slower pace of play. In response, Major League Baseball mandated a maximum break between half-innings, while instructing umpires to be stricter in enforcing speed-up rules and the size of the strike zone.[9] [10]
Although the official rules specify that when the bases are empty, the pitcher should deliver the ball within 12 seconds of receiving it (with the penalty of a ball called if he fails to do so), this rule is rarely, if ever, enforced. The umpire also has the option of calling a ball if there are runners on base, but this is also rarely, if ever, enforced. The official rules also require the batter to remain in the batter's box at all times when at bat — another rule that is "observed in the breach".

Individual and team
Baseball is fundamentally a team sport—even a franchise financially blessed enough to afford two or three Hall of Fame-caliber players cannot count on success. Yet it places individual players under great pressure and scrutiny. The pitcher must make good pitches or risk losing the game; the hitter has a mere fraction of a second to decide what pitch has been thrown and whether to swing at it. While managers and coaches can signal players to pursue certain strategies, no one can help the pitcher while he pitches or the hitter while he bats. If the batter hits a line drive, the outfielder, as the last line of defense, makes the lone decision to try to catch it or play it on the bounce. Baseball's history is full of heroes and goats—men who in the heat of the moment (the "clutch") distinguished themselves with a timely hit or catch, or an untimely strikeout or error.

The uniqueness of each baseball park

The main scoreboard at Wrigley Field. This photo was taken during the August 27, 2005 Cubs-Marlins game.
Main article: Baseball park
Unlike the majority of sports, baseball playing fields can vary significantly, within certain guidelines, in size and shape of the field. With the exception of the strict rules on the dimensions of the infield, discussed above, the official rulessimply state that fields built after June 1, 1958 must have a minimum distance of 325 feet (99 m) from home plate to the fences in left and right field and 400 (121 m) feet to center. This rule (a footnote to official rule 1.04) was passed specifically in response to the fence at the Los Angeles Memorial Coliseum, which was not originally designed for baseball, and thus was only 251 feet (77 m) to the left field pole (1 foot [0.3 m] over the bare minimum required by the rules). Major league teams often skirt this rule. For example, Minute Maid Park's Crawford Boxes are only 315 feet (96 m), and with a fence much lower than the famous "Green Monster" at Fenway Park which is labeled as 310 feet (94 m) away and 37-foot (11 m), two-inches tall. And there are no rules at all regulating the height of "fences, stands or other obstructions", other than the assumption that they exist. However, teams are required to obtain approval from the League Office when constructing new stadiums, or when proposing alterations.
Because of this flexibility, there are numerous variations in park configuration, from different lengths to the fences to uneven playing surfaces to massive or minimal amounts of foul territory. The differing styles create a unique sense of ambiance in each location, something that many fans find alluring (and even a source of civic pride). All of these factors, as well as local variations in altitude, climate and game scheduling, can affect the nature of the games played at those ballparks. Certain stadiums eventually get labeled as either a "pitcher's park" or a "hitter's park", depending on which side benefits more from the unique factors present. Some ballparks, notorious for both strong and frequently shifting wind currents, such as Chicago's Wrigley Field can be either, depending on the wind direction at any given time.
In the end, the lack of a consistent, standardized playing field has caused some debate, particularly when comparing players statistics and career records. For example, hitting a ball off the Green Monster in Boston results in a hit, where at San Francisco the same hit may have cleared the wall, resulting in a home run.