It works like a regular sorting comm, where you answer various questions and members use your answers to sort you, in this case into one of seven Great Houses of Westeros: Greyjoy, Lannister, Martell, Stark, Targaryen, Tully or Tyrell.
Once you've been sorted there are tons of things to get involved in: monthly contests at thetourneys (anything from logic puzzles to fic contests to cosplay); making and viewing creative works at valyrian_forged; shit and giggles at jesteros; epic times in your house comm (e.g. thetullys!); the occasional mammoth weekend of contests that is tourneyohand; and character stamping at westerostamping.
I was sorted around this time last year (I joined just after S2, IIRC) and since it is the best landcomm I've ever seen, I'd love to see some of you guys over there!
Coming up: I'll post my own sorting application, some of my contest entries and my character stamping, so you guys can see what it's all about :D