Professional baseball is played in leagues throughout the world. In these leagues and associated farm teams, baseball players are selected for their talents and are paid to play for a specific team or club system.
Major League Baseball in the United States and Canada consists of the National League (founded in 1876) and the American League (1901). Historically, teams in one league never played teams in the other until the World Series, in which the champions of the two leagues played against each other. This changed in 1997 with the advent of interleague play.
In addition to the major leagues, many North American cities and towns feature minor league teams. An organization officially styled Minor League Baseball, formerly the National Association of Professional Baseball Leagues, oversees nearly all minor league baseball in the United States and Canada. The minor leagues are divided into classes AAA, AA, High A, Low A, Short-Season A, Advanced Rookie, and Rookie. These minor-league divisions are affiliated with major league teams, and serve to develop young players and rehabilitate injured major-leaguers. The Mexican League is an independent league that is a member of Minor League Baseball and has no affiliations to any Major League Baseball teams. Organized Baseball is often applied as an umbrella term for all leagues -- Major and minor -- under the authority of the Commissioner of Baseball.
Operating outside the Minor League Baseball organization are many independent minor leagues such as the Atlantic League, American Association, Frontier League,Can-Am League and the feeder league to these the Empire Professional Baseball League.
During the late 19th and early 20th centuries, African-American players were barred from playing the major leagues, though several did manage to play by claiming to be Cubans or Indians. As a result, a number of parallel Negro Leagues were formed. However, after Jackie Robinson began playing with the major-league Brooklyn Dodgers in 1947, the Negro Leagues gradually faded. The process of integration did not go entirely smoothly; there were some ugly incidents, including pitchers who would try to throw directly at an African-American player's head. Now, however, baseball is fully integrated, and there is little to no racial tension between teammates.