Oh, and pretty much anything is allowed here, but for specifics look up one of the relevant threads. There's stickied threads for do's and don'ts in pretty much every subforum IIRC.

But the most important stuff is;
Don't be an asshole.
Don't harass people.
If someone's being a troll, it's best to just notify a moderator, if you 'feed' him/her you may get punished.
Always err on the side of caution if you're unsure about anything.