The World Series pits the champion of the American League against the champion of the National League to determine an overall winner. Each league is divided into three divisions each -- West, Central and East. The team with the best record in each division gets an automatic playoff berth. The team in each league with the best record that didn't win a division (the "wild card") gets the fourth and final playoff spot.
The four teams in each league are whittled down to two during the American League Division Series and the National League Division Series. The team with the best record overall is the No. 1 seed, and it faces the wild card team (the No. 4 seed). The other two division winners get the No. 2 and No. 3 seeds, based on their records. The series is best-of-five, with the first team to win three games advancing. This format has been used since 1995. MLB had expanded to 28 teams and three divisions in each league by then. Prior to that, each league had two divisions, and division winners played each other in the League Championship Series (LCS). The divisional playoff was pioneered in the strike-shortened 1981 season, in which champions of each division's first and second halves played for the right to go to the LCS. The first two games of the Division Series are held at the home of the team with the better ranking, and the next two at the other team's home, with the fifth game, if necessary, back at the higher-finishing team's home field.
The winner of the 1-4 divisional matchup then plays the winner of the 2-3 matchup in their League Championship Series. This is a best-of-seven matchup, meaning the victor is the first team to win four games. As in the Division Series, the first two games are held at the better team's home, but then three games are at the other team's home, before shifting back to the higher ranking team's home for the last two games, if necessary.
The National League winner then faces the American League winner in the World Series. These League Championship series date to 1969, when each league expanded to two divisions. Before that, the team with the best overall record in each league went directly to the World Series. The same 2-3-2 home field scheme from the LCS is used for the World Series, but home field advantage is secured at that season's All-Star Game. If the National League won that year's All-Star Game, for example, then the NL team in the World Series gets home field advantage, regardless of regular season record. This rule change was enacted in 2003, in order to make the All-Star Game more than just an exhibition game.
The World Series isn't just for fun – sometimes it makes the news. Read on to see how.