I know that there are a lot of strange terms used in Gundam Wing fanfiction. I do not know if they extend into other series fictions as well, but I *do* know that *these* are the commonly accepted terms in the realm of Gundam Wing.
Shonen-ai: Literally translated, "boy-love." Guys have crushes on guys in these fics: whether or not it works out is up to the author, like everything else.
Shojo-ai: Similar to "shonen-ai" except that it's girls crushing on girls.
Yaoi: Serious romantic male/male relationships.
Yuri: Serious romantic female/female relationships.
Het: Short for "heterosexual." Whatever goes on romantically, it's boy-girl and/or girl-boy.
Lime: Sexual activity is present, but is implied or described in vague euphamisms.
Lemon: Graphic sexual activity: these are X-rated, NC-17. If you're underage, pleae do not read these. If you're underage and you still decide to read these, just don't let your parents find out!
PWP: Short for "Plot? What plot? There *is* no plot, just sex!"
TWT: Short for "Timeline? What's that?"
Angst: Emotional and/or physical trauma.
Silly, Fluff: This doesn't need explanation, does it?
Language: Swearing, cursing and/or cussing is frequent here. (Yes, there is a difference!)
Humor: Now *this* should not need an explanation!
Violence: Blood, gore, ugly scenes...
OOC: Short for "Out-Of-Character."
XO: Crossover: characters and/or scenes from another series (not necessarily anime) are involved.
Death: Someone's going to die...
Songfic: This is a fic that uses song lyrics from an actual song to help tell the story. If you do one of these, please do tell us what the song is!
Squick: Oh, boy, I love these. These ae the ones that are pretty much guaranteed to gross *somebody* out: if you can handle it, good for you!
If there are any more warnings that you know of, please do not hesitate to let me know of them!