The pitcher (aka hurler or moundsman) initiates play by throwing the ball for the batter to hit. Good pitching is so important in defense that the role of the pitcher is qualitatively different from that of all other defensive players. Unlike every other position, where even sterling defenders are expected to pull their weight offensively, pitchers are selected without regard to their ability as batters.
The physical strain of pitching also makes it qualitatively different from all other positions. The pitching motion is unnatural and places extreme strain on the pitcher's arm. Unlike other positions where a single regular player receives the lion's share of a team's playing time, it's rare for a contemporary pitcher to throw more than about 1/6 of his team's innings.
Because each pitcher throws such a small fraction of his team's innings, it's necessary for each team to have a pitching staff consisting of as many as 12 or 13 pitchers. Each pitcher is assigned a role, generally either as a starting pitcher or as a relief pitcher.
The starting pitcher is, as the name implies, the pitcher who starts the game for his team. The starter is expected to pitch deep into the game, and usually won't be replaced until he tires or needs to be replaced with a pinch hitter. Starting pitchers commonly throw 100 or more pitches in a game and require several days to recover from the effort. Contemporary teams typically have 5 starting pitchers who take turns in a pitching rotation.
A relief pitcher takes over when the starter gets tired, pinch hit for, or pulled from the game for poor performance. Relievers don't just pitch until they're tired. Instead, relievers are given specialized roles that call for them to pitch a limited amount before being replaced by another reliever. That allows relievers to throw more often than starters, even in several games in a row, but their total number of innings is generally less than that of any starter. Additionally, while starting pitchers generally try to employ a wide bevvy of pitches to keep batters off-balance over multiple innings, relievers tend to use only their strongest pitches (usually between 2 and 4) to make the few outs they are responsible for.
A long reliever is the exception to the rule that relievers pitch only in limited doses. A long reliever is brought in when the manager needs to get several innings of relief, either because he was forced to pull the starter early in the game or because the game is in extra innings and he doesn't know how long it will last. The requirements of a long reliever are more similar to those of a starting pitcher than to other relievers, and manager Earl Weaver said that long relief was an ideal role for a young pitcher trying to break into the starting rotation. A long reliever brought in when the game seems hopelessly out of reach is known as a mop up man.
A middle reliever is brought into the game later than a long reliever would be, usually in the 6th or 7th inning. He is expected to protect a lead, or prevent the other team from extending its lead, for about one inning before giving way to a setup man. Middle relievers are often brought in when the starter has faltered and allowed batters to reach base, so they often must be good at stranding inherited runners.
A setup man is usually brought into the game in the 7th or 8th inning when his team is tied or has the lead. He is expected to protect the lead or the tie before the manager brings in his closer. Setup men are often viewed as closers-in-waiting.
The closer is a reliever who specializes in finishing the game, and the position is normally held by the best reliever on the staff. Modern usage calls for the closer to be brought into the game to start the 9th inning when his team has a lead of 3 runs or less.
A LOOGY (short for "Lefty One Out GuY", aka lefty specialist) is a reliever who is brought in to face one or two left-handed batters at a critical juncture of the game. Some teams have right-handed pitchers who are used in a similar role against right-handed hitters, but their use is much less stereotyped than the use of lefty specialists.
A swingman (aka spot starter) is a pitcher who may be used either as a starter or as a reliever. The requirements of a swingman are similar to those of a long reliever, and the long relief role is where he would most commonly be used when relieving. The swingman will be given an occasional start, either to give the rest of the staff an extra day of rest, to spell a starter who must miss a start because of injury or suspension, or because the team has a doubleheader.
The early season schedule has more days off than the rest of the year, so it's often possible to skip one spot in the rotation while still giving the rest of the starters their customary rest. Some teams will take advantage of this by moving their 5th starter into a swingman role for the first month of the season.
The extreme specialization described above applies only to recent usage. Pitching usage has evolved over time, with trends toward larger pitching staffs and increased specialization. A brief summary of changes in pitching usage:
- In pre-professional and National Association times, teams relied on a single pitcher with only occasional relief appearances, usually by position players. This was possible (and necessary) because teams played only a few games a week, pitchers were restricted to a less stressful underhand motion, and substitutions required the consent of the opposing team.
- The schedule expanded rapidly from the founding of the National League to 1892. Pitchers were also allowed to use more effective, but more stressful, sidearm and overhand pitching motions. These developments made it impossible for teams to rely on a single main starter, so they expanded their staffs to 2 and then 3 main pitchers. Relief appearances were still rare, though replacement rules made it possible to use other starting pitchers as relievers rather than position players.
- The pitching distance was increased in 1893, and staffs had to grow again as pitchers strained harder. By 1900, most teams spread their starting workload among 4 or even 5 pitchers. Many teams had a handful of pitchers who were used mostly in relief, but relief appearances were still rare enough that the relief workload was very light.
- During the Deadball Era, rosters expanded, and pitching staffs followed suit. Teams still spread their starting workload among 4 or 5 pitchers, but relief appearances became much more common. Some teams begain to experiment with true relief pitchers.
- In the 1920's and early 1930's, teams expanded their starting corps even further, and often had as many as 6 pitchers who got significant numbers of starts. Teams continued to experiment with full-time relievers, but most pitchers both started and relieved.
- In the late 1930's and 1940's, full-time relief pitchers became common. Managers became more aggressive about bringing in relievers to protect leads, rather than just replacing pitchers who had been pinch hit for or knocked out of the game.
- In the 1950's through 1970's, improved transportation allowed more regular schedules, which let teams settle on a 4 man rotation. The distinction between starters and relievers really solidified. The number of complete games continued its long term decline. Teams had recognized ace relievers who would be used in the toughest situations, but usage wasn't highly specialized.
- In the late 1970's and 1980's, teams began to switch to 5 man rotations instead of 4 man rotations. The number of complete games continued to decline. Managers became worried about their top relievers burning out and tried to restrict their usage to protect their arms. The older relief ace who was used in any tight situation was replaced with something more like the modern closer, who was used only to protect leads late in the game.
- From the late 1980's to today, teams rapidly expanded their number of relief pitchers and gave them more specialized roles. The 5 man rotation became universal, and complete games all but vanished.
- Thomas Boswell: "Arms and the Men", in How Life Imitates the World Series, Penguin Books, New York, NY, 1982, pp. 191-195.
- Philippe Cousineau: "Pitchers in the Field: The Use of Pitchers at Other Positions in the Major Leagues, 1969-2009", The Baseball Research Journal, SABR, Volume 40, Number 2 (Fall 2011), pp. 84-88.
- Glenn P. Greenberg: "Does a Pitcher's Height Matter?", The Baseball Research Journal, SABR, Vol. 39, Number 2 (Fall 2010), pp. 51-57.
- Lou Hernandez: Baseball's Great Hispanic Pitchers: Seventeen Aces from the Major, Negro and Latin American Leagues, McFarland, Jefferson, NC, 2015. ISBN 978-0-7864-7975-7
- Donald Honig: The Greatest Pitchers of All Time, Random House, New York, NY, 1988. ISBN 051756887X
- Bill James and Rob Neyer: The Neyer/James Guide to Pitchers: An Historical Compendium of Pitching, Pitchers, and Pitches, Fireside Books, Simon & Schuster, New York, NY, 2004. ISBN 978-0743261586
- Roger Kahn: The Head Game: Baseball Seen from the Pitcher's Mound, Harcourt, Inc., New York, NY, 2000. ISBN 0156013045
- John A. Knox: "The 100 Top-Fielding MLB Pitchers, circa 1900-2008", in The Baseball Research Journal, SABR, Volume 38, Number 1 (Summer 2009), pp. 49-58.
- Norman L. Macht: "Five Reasons Why Pitchers Suffer Arm Injuries", Baseball Digest (March 1991), pp. 57-58. 
- Terry McDermott: Off Speed: Baseball, Pitching, and the Art of Deception, Pantheon Books, New York, NY, 2017. ISBN 978-0307379429
- William F. McNeil: The Evolution of Pitching in Major League Baseball, McFarland, Jefferson, NC, 2006.
- Bob Nightengale: "As MLB celebrates Jackie Robinson, dearth of black pitchers concern many", mlb.com, April 14, 2016. 
- Jeff Passan: The Arm: Inside the Billion-Dollar Mystery of the Most Valuable Commodity in Sports, HarperCollins Publishers, New York, NY, 2016. ISBN 9780062400369
- J.G. Preston: "How Did That Guy Do That? The Unlikeliest Pitching Performances in Major League History", The Baseball Research Journal, SABR, Volume 44, Number 2 (Fall 2015), pp. 5-9.
- David Vincent: "Pitchers Dig the Long ball (At Least When They Are Hitting)", The Baseball Research Journal, SABR, Volume 41, Number 1 (Spring 2012), pp. 7-11.
- Rich Westcott: Great Stuff: Baseball’s Most Amazing Pitching Feats, Sports Publishing LLC, New York, NY, 2014.
- Warren N. Wilbert: What Makes an Elite Pitcher? Young, Mathewson, Johnson, Alexander, Grove, Spahn, Seaver, Clemens, and Maddux, McFarland, Jefferson, NC, 2003. ISBN 978-0-7864-1456-7
|Outfielders:||Left field | Center field | Right field|
|Infielders:||3rd base | Shortstop | 2nd base | 1st base|
Pitcher | Catcher