If you didn't follow the link above and/or read the spoiler from that thread, IGYNPADCA stands for Incomprehensible Game You've Never Played And Don't Care About. The idea is that we pretend that this is a game that actually exists, but really we're all just making everything up as we go along. And we never break character about that fact outside of OOC spoilers.
Now for this game in particular, what I've said above is essentially true, but, in addition to that, you'll also get a bonus to your rolls if you come up with a creative description of how you're abusing game mechanics to perform an action. The more entertaining, the better (as in, you'll get a higher bonus).
Your class/race/whatever-other-aspects-of-your-character-build has little mechanical effect, other than as a potential source of flavor for your actions, and you can do whatever you want as far as your character sheet goes, but I ask that you not include anything that would violate trademark or IP laws if it was included in an actual game, as that hurts realism and I will pass over any such character (and the same will go for anything you come up with in the game in general). So no TARDISes or Cylons or Hobbits, though you can have
expies or whatever of such things as much as you want, and your character could be named Legolas or Harry Potter etc., since IC that's player-created content, which no one really cares about legally-speaking. Likewise, there might be fan nicknames for things which don't pass legal muster, and also might include obscenities and other objectionable material that no sane company would every include or endorse in a game.
You don't really need to sweat too much about the IP stuff, though. I mean, keep it in mind, but it's okay if something slips past you, since GGC (Generic Game Company, the supposed developers of IGYNPADCA) are somewhat canonically supposed to be a rather ramshackle enterprise, as far as I recall, and if someone notices such a thing we can easily lampshade it so that it's something that so far has gone unnoticed or has been noticed and is the subject of some legal battle (or we could just say whoever mentioned it is using a slang term).
And I think that's about it for what I have to say about this game. Have at it.