It's a league run by TheGhostmaker (one of the mods) that has a number of levels to climb to reach the top (Premiere, A, B, C, and D). At each level, there are a different number of individual leagues of seven players. Premiere and A leagues each have one league of 7. B has two leagues, C three, and D four (sometimes 5, depending on demand). Win you league and you move up to the next level, lose and you move down. And, it isn't just the one winner and the one loser (as that wouldn't mathematically work), but a number of losers move down and, in some cases, best of the second place winners move up.
Check out http://tournaments.webdiplomacy.net/thephpleague for all the details and to see the current list of who's who in which leagues.