From BR Bullpen
The Postseason refers to games played after the end of the regular season to determine the league's champion. Between 1884 and 1890 postseason games were played only as exhibition matches. The first official World Series between the American League and the National League took place in 1903. There was no such postseason in 1904, however the tradition took on more permanence as of the 1905 season.
Up to 1968, the only postseason series was the World Series, featuring the teams who won their respective league pennants. The leagues were not yet divided into Divisions. The winners of the titles had to have the best overall record in their league. Those titles were sometimes decided by a one-game or three-game playoff, but these games were counted as part of the regular season.
With the expansion of 1969, the leagues were divided into Eastern and Western Divisions, necessitating League Championship Series, which became the first round of the postseason. From 1969 to 1984 these series were held as best-of-five playoffs, and from 1985 until 1993 they were best-of-seven playoffs. The two winners would then meet in the World Series, which became the second round of the postseason.
There was only one exception to this scenario, and it came in the 1981 Postseason. It was caused by the 1981 strike. The strike came in the middle of the season, so a decision was made after the strike was resolved to have the leading teams of each division from before the strike - the so-called "first-half champions" - play the leaders of each division from after the strike - the "second-half champions". See 1981 Split Season Schedule.
In 1994, the leagues were further divided into three divisions: Eastern, Central, and Western. In order for postseason play to produce a winner in an even number of match-ups, a Wild Card winner was chosen from each league. This would be the team from each league who had the best overall record yet did not finish first in its division. The three division winners along with the Wild Card winner from each league would play a five-game Division Series. Ironically, the first year of this new arrangement did not go as planned. The 1994 strike cut the season short and postseason play was cancelled for the first time in 90 years. The first such postseason scenario took place in the 1995 Postseason.
In 2012 the postseason was expanded again with the creation of a second wild card team in each league. The now two wild card teams per league would play each other in a one-game winner-take-all round hosted by the team with the better record - the Wild Card Game. The winner would then advance on to the Division Series to play the team with that league's best regular season record, regardless of division.
As a result there are now four rounds of postseason play:
- The Wild Card Game
- Best of 5 Division Series
- Best of 7 League Championship Series
- Best of 7 World Series
 Further Reading
- Stuart Shapiro: "Measuring Franchise Success in the Postseason", The Baseball Research Journal, SABR, Volume 44, Number 2 (Fall 2015), pp. 32-36.