1. Major League Baseball
Major League Baseball / MLB consists of the National League and the American League. They play by the same rules except that in the National League the same nine players have to play both offense and defense (unless and until they are substituted for). In the American League a team can (and, 99.9 % of the time, does) designate a tenth player—the designated hitter—to be the batter whenever it would be the pitcher's turn. This rule was intended to increase the offensive action of the game. Pitching is such a valuable skill that its practitioners don't have to be good batters. For simplicity's sake we follow National League rules in this discussion.
2. Minor Leagues are professional ... MLB gets players from them.
3. College Leagues
4. USSSA Leagues
5. Dixie Youth Leagues
6. Pony Leagues
5. Babe Ruth Leagues
6. Cal Ripkin League is an affiliate of Babe Ruth, but for kids 8-12
6. Little League is also for 8-12 year olds