From BR Bullpen
A Platoon is when two players share a position. The most common platoon uses a left-handed batter against right-handed pitching and a right-handed batter against left-handed pitching. Other kinds of platoons, like a good fielder/weak hitter and a weak fielder/good hitter are possible, but it's safe to assume a left/right platoon unless otherwise specified.
The key to a traditional left/right platoon is that most batters hit better against opposite-handed pitching than against same-handed pitching. There are several reasons why this is the case. One major reason is that batters have an easier time seeing the ball when it's thrown by an opposite-handed pitcher. Another important reason is that breaking balls tend to break away from same-handed batters and in toward opposite-handed batters, and most batters have an easier time adjusting to inward than outward break.
The difference between a batter's performance against opposite handed pitching and same handed pitching is his platoon split; a right-handed batter who has a .300 batting average against lefties and .250 against righties has a .050 platoon split for batting average. A batter whose platoon split is negative has a reverse platoon split. A batter with a normal platoon split who is hitting against an opposite-handed pitcher has the platoon advantage; in contrast, if both the batter and the pitcher are working from the same side, the pitcher is the one who has the advantage.
Baseball players have understood the advantages of platooning from the very early days of the game. Switch hitting, a strategy that only makes sense in light of the platoon advantage, was in use from the beginning of the National Association. Although players understood that they had an advantage against opposite-handed pitching, early teams' rosters were too small to take advantage by having formal platoons.
The first really noteworthy example of platooning was the 1914 "Miracle" Braves. The Braves took advantage of enlarged rosters to platoon at several positions in the outfield, and unexpectedly won the pennant and the World Series. Braves' manager George Stallings was hailed as a genius, and platooning swept through baseball. Despite its advantages, platooning became less popular in the 1930s and 1940s until it was reintroduced by Casey Stengel of the New York Yankees. Not surprisingly, Stengel, as a lefthanded-hitting outfielder, had himself been platooned during the 1920s, a strategy which worked very well. Stengel's time with the Yankees was when the strategy got its name. While platooning's popularity has waned and waxed since Stengel's day, it has never gone away completely.
Nowadays, platooning occurs most often between pitchers, as managers will switch once or twice an inning late in the game from a righthanded pitcher to a lefthander in order to gain the platoon advantage, while the batting team often has few opportunities to make a counter-move, because there are fewer substitute position players available. Most teams now have a couple of LOOGYs on their roster to implement this strategy, which was made popular by Tony LaRussa.
 Further Reading
-  Article on Platoons in The Hardball Times, part 1.
-  Article on Platoons in The Hardball Times, part 2.
-  Article on Platoons in The Hardball Times, part 3.
-  Article on Platoons in The Hardball Times, part 4.
-  Article on Platoons in The Hardball Times, part 5.
- Bryan Soderholm-Difatte: "The 1914 Stallings Platoon: Assessing Execution, Impact, and Strategic Philosophy", The Baseball Research Journal, Vol. 43, number 2, Fall 2014, pp. 18-28.