From the introduction:
Philosophy ain't Just a Fact of Life...
...it's a Way of Life, in Sigil for starters, and even more so in the rest of the Outer Planes. The Factions are a group of fifteen organised philosophies (though some'd dispute even that) based in Sigil. That's really the only requirement to being a faction: a representation in the Cage. There haven't always been fifteen; long ago there were many more, only the Lady of Pain didn't seem to like it that way. Since her decree, no cutters've dared barge their philosophy into Sigil and exceed the number, though the Lady never specified her objection was permanent. Recently, it looks like it might be all-change anyway: it's common chant that the Ring-Givers have their eyes on the Greatest Ring of All... Sigil.
The factions ain't static, carved-in-stone things. They're living, breathing, emotional creatures. Yeah, even the Fraternity of Order! Recent big news and new facets of the factions are also described here, be they prominent members, plots, groups or threats to factioneers. There's also chant to help a prime tumble to the dark of the Way Things Are in the cut-and-thrust world of planar philosophy...