They are often called pyramids, due to their tendency to split into an increasing number of regional divisions further down the system.
In association football, rugby union, rugby league and Gaelic games, league systems are usually connected by the process of promotion and relegation, in which teams from a lower division who finish at the top of the standings in their league are promoted (advanced to the next level of the system) while teams who finish lowest in their division are relegated (move down to a lower division).
In North America, league systems in the most popular sports do not use promotion or relegation.
Different professional leagues play by very different sets of rules in different seasons (the NFL plays 11-a-side on a 100-yard field in autumn and early winter, the CFL uses 12-a-side on a 110-yard field in summer and early fall, while arena football and the minor indoor leagues each play 8-a-side on a 50-yard field in the spring and early summer).
League systems can have a negative effect on competitive balance, particularly in European professional football.