Major League Baseball is the professional baseball league in the US and Canada, and is made up of the National League and the American League. Prior to 2000, the National and American leagues technically functioned as two separate entities. However, their relationship has been close for a long time; since at least 1903 when they started meeting for a season ending championship series, the World Series. The first formal introduction of a partnership was in 1920 formation of the National Commission, which was eventually replaced by the Commissioner of Baseball. The current commissioner, Bud Selig, helped bring the American and National League closer together, and helped introduce inter-league play, which truly made Major League Baseball one league.
The American and National leagues were not always the only professional leagues in the US. The most popular competitor was the Federal League which disbanded in 1916. The only remnant is Wrigley Field, which was originally built for the Chicago Whales, a Federal League team. There are currently 30 teams in Major League Baseball, 15 in each league, each divided into three, five team divisions. The playoff structure allows 4 teams from each league to make the playoffs, with the eventual winner from each league playing in the World Series to determine that season's champion.