Baseball, like most other popular sports, developed over a period of time with rules that became systematic in order to allow organized competition. Abner Doubleday is generally credited with codifying rules to create a game which, after a century and a half, would still be recognized as baseball by modern fans.
Professional baseball developed before there were leagues. The two "major" leagues of baseball have dominated the sport since 1900. The National League was formed in 1876 and the American League, formerly a minor league known as the Western League, which claimed major league status in 1901.
Since 1903, the champions of the two major leagues have played in the World Series at the conclusion of the regular season. The designation is considered odd in most of the world, since baseball is a game almost entirely played in the North America, with the addition of a few countries in which American influence has been strong, such as Japan and Taiwan.