There are nine players on each team during a baseball game, however, baseball rosters usually include 25 players so that those nine players may rotate within the game and from game to game throughout the baseball season. Player rotation is very strategic because coaching staff will carefully examine the team's schedule to determine which players should play which games. Within games, players may be substituted, but these are permanent substitutions, meaning that the player who is substituted out of the game may not return for the remainder of the game.
The following descriptions apply to the nine players' positions on defense:
- Catcher (C): This player lines up behind home plate and catches the ball that is thrown by the pitcher. He also has the best vantage point of the entire field, so he typically calls the pitches using hand signals. He is also the last line of defense in getting a player out before he reaches home plate. Famous catchers include Buster Posey (San Francisco Giants), Yadier Molina (St. Louis Cardinals), and Jonathan Lucroy (Milwaukee Brewers).
- First Baseman (1B), Second Baseman (2B), and Third Baseman (3B): These players line up near their respective bases on the baseball diamond and try to get out base runners. The 1B and 3B in particular must have quick reflexes to try to catch balls that are hit along the foul lines. Third base is often referred to as "the hot corner" and first base is "the other hot corner."
- Shortstop (SS): These are players that line up between second and third base. They specialize in defense, so they are usually fairly poor batters and will line up near the end of the batting order.
- Famous infielders include Miguel Cabrera (1B, Detroit Tigers), Albert Pujols (1B, Los Angeles Angels), Robinson Cano (2B, Seattle Mariners), Adrian Beltre (3B, Texas Rangers), Troy Tulowitzki (SS, Toronto Blue Jays), and Jose Reyes (SS, Colorado Rockies).
- Left Fielder (LF), Center Fielder (CF), and Right Fielder (RF): These players will line up farthest from the batter, in the grassy outfield to catch fly balls (balls hit high in the air). The CF is generally the fastest and most agile because he has to cover the gaps in the center of the field.
- Famous outfielders include Bryce Harper (Washington Nationals), Mike Trout (Los Angeles Angels), and Andrew McCutchen (Pittsburgh Pirates).
- Pitcher (P): The player who throws or "pitches" the ball towards the catcher with the goal of striking out the batter. He is often considered the most important defensive player. There are starting pitchers (those that begin and throw for a majority of the game) and relief pitchers (those that enter the game after the first pitch to relieve the starting pitcher). Famous pitchers include Clayton Kershaw (Los Angeles Dodgers), Max Scherzer (Washington Nationals), and David Price (Boston Red Sox).
- Starting pitchers: These pitchers throw for a long portion of the game and require rest between games, so there are usually 4-6 starting pitchers so that they can be rotated throughout the season. The best starting pitcher is called the ace.
- Relief pitchers: These pitchers enter the game strategically, late in the game, after the starting pitchers start to fatigue. There are a few different types of relief pitchers including:
- Closer (CL): Usually the team's best relief pitcher, who specializes in getting batters out in the final innings.
- Setup Relief Pitcher: A pitcher who regularly pitches after the starting pitcher and before the closer. This is usually the second best relief pitcher, and may get promoted to closer.
- Middle Relief Pitchers: A pitcher who will relieve the starting pitcher to play a few innings, and will likely be relieved by another relief pitcher. He may continue to pitch for some of the later innings if the game is not close.
- Left-Handed Specialist: This pitcher throws left-handed and specializes in pitching to left-handed batters. He may also throw to poor right-handed batters.
- Long Reliever: A pitcher who enters the game in an early inning of the game, if the starting pitcher must leave the game early.
The team's same nine players must bat when the team is on offense. The players will cycle through a particular batting order, with each player batting once and recycling back through in the same order, whenever the team is on offense.
At times, a team may permanently substitute a pinch hitter for a player in the batting order. The pinch hitter is usually a back-up infielder or outfielder that will be used in the later innings of the game to substitute for a pitcher, because pitchers are usually some of the worst batters. In the American League (one of Major League Baseball's two conferences), designated hitters are allowed to bat in place of the pitcher in the batting line-up throughout the game, for exactly this reason. However, designated hitters are not permitted in the National League (the other MLB conference), so they must carefully substitute in pinch hitters.