A poker league can be defined simply in one sentence, but capturing the full meaning of the term takes a bit longer to do. For example, one might say that a poker league is a gathering of different poker players in which players play individually or in groups in order to win the league and get the prize. This definition, while not particularly detailed, gets the main point across which is that the poker league concept turns poker into a competition akin to bowling or any team sport in which the main goal is not individual achievement of victory, but rather the team or your own place within the larger league structure being as large as possible.
With individual leagues, all that is needed is enough individuals for the league to be kept interesting. Indeed, individual poker leagues are often run like long poker tournaments in which the players gather together every week and play a tournament, with players gaining points towards their league table based on their performance within the individual tournaments. After a certain number of weeks, the playoffs are either decided or alternatively the people at the top of the league charts end up receiving the awards. If you are a fan of poker television programs, this description might very well remind you of Poker Superstars, which was one of the most famous televised poker shows. It followed a format that could very easily be considered a poker league.
That having been said however, poker leagues are not really associated with individuals in their colloquial use, but rather are associated instead with teams. This means that people get together into teams and then play games against other teams with the purpose of making the playoffs or winning the league outright if there is nothing available past the round robin stage of the league. Leagues can be run as substitution leagues in which each team can replace players throughout the tournament and each team plays through one seat at the poker table, or they can be run on a sum of individual achievement in which team players play at different tables and each player contributes points to their team based on their own table performance.
These types of poker leagues are not that popular yet online, but are becoming popular as people realize how much fun is out there for them to have through these types of league events. Poker leagues allow you to make poker into a team activity with your friends instead of play against them and for that reason it holds a large amount of wonder for many of the people around. Ultimately, one might expect poker leagues to become the dominant form of poker entertainment online, but that time is still likely a long way off judging by the present moment in time.