I've decided to put a warnings page on the site, in the off chance that someone new to the fanfiction arena stumbles in here and doesn't know what all this jazzy stuff means. I thought about when I was a newbie and decided to return the favor other webmasters did for me. We'll start from the small stuff, and work our way up.

Labels:

IC

The IC stands for "in character." What this means is that the character is acting how he/she would act in the series. If in a fic Heero tears up an invitation given to him, that would be considered IC because that's what he did in the series. I hope that example doesn't sound too stupid.
OOC
This is "out of character." The behavior will not be consistent to how the character would behave in the series. If in a fic Heero and Wufei put on pink leotards and pranced around in ballet slippers on an iced over lake with flowers in their hair, they're acting out of character.
Alternate Universe
Also labeled as AU. This means that the GW characters won't either entirely be in their correct timeline or in a different one altogether. For example, the cast being in the wild west, or in their own timeline but with werewolves and witches thrown in are considered AU.
Humor
This isn't really necessary, unless you have a really, really weird sense of humor, like I do. ^__^
Oneshot
This is a story that is told in one part.
Songfic
This is a fic set around the lyrics to a song.
Vidfic
This is a fic written to be read as a video. You won't find me doing these. There are some good ones out there, but I find that they're a pain in the ass to read/write. (Just my opinion though).
Sap
The fic contains so much mushy sweetness and stuff, it makes you either have a toothache, or hurl from it all.
PWP?
Stands for plot? what plot? There's no plot (or very little), and the characters end up having sex.
Angst
This is where there is going to be some serious trouble in the fic. It's mostly pain and torture of the emotional kind, like....Wufei dealing with Treize's death would be angst, because he's suffering emotionally. Another example would be when Quatre was trying to find Trowa in space after the ZERO incident and his guilt over nearly killing him.
Dark/Dark Angst or maybe Death
I've seen fics labeled in those three ways. This is basically a fic that either is extremely dark or could possibly have at least one of the main characters dying. This one can be a real pain in the ass. Why? Some authors (myself included) will have a fic with death in it, but not label it as that. We will label it as "dark" or "dark angst." That doesn't mean ALL authors do, but some do. If you do see a fic labeled as "dark," you might want to run it across with the author just to make sure if you're adamant about that type of thing.
Shounen ai
This literally translates out to "boy's love." Shounen ai is when, say, Trowa and Quatre love each other (in a non-familial sense) but don't take it to a physical level. It's just implied that they want to be together. Shojo ai is the female love version.
Yaoi
This is the next step from shounen ai. In yaoi, it's a male/male relationship that is sexual in nature. Meaning, the guys kiss, hug, and sleep with each other. The female version of this would be yuri. Yuri is really not my cup of tea, so you won't be seeing that around my site. I just wanted to mention it for your information.
Non Consensual Sex (NCS)
This is RAPE! Most authors will label it as NCS, so if you see that, it means rape. This really isn't my cup of tea either, but you never know.
Squick
This is a warning that means major, major, kinky freakiness, okay? If Trowa decides to have a little frisky fun with the lions and elephants after the show, you HAVE to put this up. Even that's a little rich for me.
Lime
Lime is saying that the story has sex, but it's not graphical. Kissing, hugging, and other sappy shit fall under this category. But this is also relative to the writer/reader. What I consider a lime can be very different than what someone else considers a lime, so if you're unsure, ask the author.
Lemon
Now this one is a real pisser when writing. A lemon is a fic/story that has sex in it, BUT has it in great detail, such as "Duo grabbed Heero's cock and gave him a hummer," alright folks? I'm sorry that sounded crude, but I don't want any misconceptions. A great deal of readers are under the age of 18, and like to read lemons, then bitch about how gross it is (especially when it's yaoi.) You'll really want to look for this if you're against lemons or a minor.

Numbers:

The names for the characters are actually numbers in different languages. So it goes:

1=Heero
2=Duo
3=Trowa
4=Quatre
5=Wufei
6=Zechs Marquise/Miliardo Peacecraft
7=Septum
8=Otto
9=Lucrezia Noin
11=Lady Une (it actually means 1, but...I don't know. *shrugs*)
13=Treize

Signs:

By now I'm sure you've seen the +/x signs between two numbers or letters. What the "+" sign means is that there may be romance between the characters, but it's not taken to a physical level. They may just be friends, kiss, other mushy stuff, but that's about it. The "x" sign means that they're more than just friends, but not necessarily sex in the relationship.
Now for the order:
Whatever comes before the sign, for example 1x2, that means that whoever is in the front dominates the relatioship, and whoever's behind the sign is submissive. For yaoi, it's basically whoever's on top most of the time. Now, if it is a heterosexual couple, the male will always be first, like 5xSally. Sorry ladies, I know it bites. But think about it. Wouldn't you want Wu on top?

The final label would be both of the signs being shown, like 1x2/2x1. In that case Heero nor Duo dominant. Both parties participate equally in the relationship.

There you are folks. That's it in a nutshell.