I heard about this in passing, and there are a LOT of versions of these Rules floating around, but this was the most definitive and common set I could find. Some sites list many times more, and some are good, but let's just keep it simple:
1. Do not talk about /b/
2. Do NOT talk about /b/
3. We are anonymous
4. Anonymous is legion
5. Anonymous never forgives
6. Anonymous can be a horrible, senseless, uncaring monster
7. Anonymous is still able to deliver
8. There are no real rules about posting
9. There are no real rules about moderation either – enjoy your ban
10. If you enjoy any rival sites – DON’T
11. All your carefully picked arguments can easily be ignored
12. Anything you say can and will be used against you
13. Anything you say can be turned into something else – fixed
14. Do not argue with trolls – it means that they win
15. The harder you try the harder you will fail
16. If you fail in epic proportions, it may just become a winning failure
17. Every win fails eventually
18. Everything that can be labelled can be hated
19. The more you hate it the stronger it gets
20. Nothing is to be taken seriously
21. Original content is original only for a few seconds before getting old
22. Copypasta is made to ruin every last bit of originality
23. Copypasta is made to ruin every last bit of originality
24. Every repost is always a repost of a repost
25. Relation to the original topic decreases with every single post
26. Any topic can be easily turned into something unrelated
27. Always question a person’s sexual preferences without any real reason
28. Always question a person’s gender – just in case it’s really a man
29. In the internet all girls are men and all kids are undercover FBI agents
30. There are no girls on the internet
31. TITS or GTFO – the choice is yours
32. You must have pictures to prove your statements
33. Lurk more – it’s never enough
34. There is porn of it, no exceptions
35. If no porn is found at the moment, it will be made
36. There will always be even more fucked up shit then what you just saw
37. You can not divide by zero (just because the calculator says so)
38. No real limits of any kind apply here – not even the sky
39. CAPSLOCK IS CRUISE CONTROL FOR COOL
40. EVEN WITH CRUISE CONTROL YOU STILL HAVE TO STEER
41. Desu isn’t funny. Seriously guys. It’s worse then Chuck Norris jokes.
42. Nothing is sacred
43. The more beautiful and pure a thing is – the more satisfying it is to corrupt it
44. Even one positive comment about Japanese things can make you a weeaboo
45. When one sees a lion, one must get into the car.
46. There is always furry porn of it.
47. The pool is always closed.
OK, now what I can fill you in on, if you don't know:
* /b/ is an image board, like a message board, but with just images. It's ran by a site called 4chan. In addition to starting all kinds of Internet fads, the reason "we don't talk about /b/" is that from time to time, a lot of child porn gets posted up there. Since 4chan takes extreme measures to keep members anonymous (and it allows anonymous posting) this goes unpunished. 4chan eventually deletes the porn, but their site is notorious for trafficking child porn. Hence two rules.
* 3-7 also apply to 4chan, and any other site that allows anonymous posting.
* 9 is the oldest misunderstanding in the book, that free speech applies to forums. It does not and you can be banned for any reason. It's not illegal. Re-read the bill of rights. (Hint: the message board that banned you ain't Congress.)
* 10 pretty much means that sites don't like competition. Naming other sites you visit is often taboo.
* 11-21 are general rules for how forums operate.
* Copypasta is shorthand for "copy and paste". Figure it out.
* 29 is my favorite. 30's cool too.
* TITS stands for, well, tits. Breasts. GTFO on the other hand stands for "Get the fuck out". If you don't have anything to add, post tits to justify your presence or leave. Unless you're on a kiddy board like GameFAQs where you can't post tits.
* 32. Commonly enforced. Usually with the text "pics or it didnt happen". Usually after someone tells a story involving a female, especially if it's a sister, even if it's a kid, and if it's a girlfriend, she's gotta be holding a sign with your screen name on it and a statement like "I love so-and-so".
* 33 - Lurking means reading posts without participating, so as to get a feel for how a community works.
* 34 is how I got introduced to the Rules. Search Google for "Rule 34" with SafeSearch turned off. Chances are you won't see anything too dirty, just funny.
* 39 refers to idiots who think they'll be answered faster because they typed their ENTIRE MESSAGE IN ALL CAPS. David Draiman of the rock band Disturbed did this on their forum when it was open to the public, and those who knew better secretly laughed at him for it.
* A weeaboo is someone who isn't Japanese but wishes they were, usually because they love anime.
And I think that's about all I can answer. Got more rules? Go ahead and leave 'em in the comments.