x – a pairing indicator. Refers to sex between characters in the listed pairing. The first number or name is the seme, the second is the uke. Like this: SanoxKenshin, HeeroxDuo (1x2). In the case of a heterosexual pairing, the male should always come first, because he penetrates.
AN or A/N - author's note. Generally at the beginning of the story or chapter, and sometimes at the end. These should never be placed in the middle of the story.
Angst - a category of fic that includes sad feelings and/or otherwise painful emotions. It can mean physical and/or emotional pain, and may induce tears.
AU or A/U - alternate universe. The characters will be taken from their original settings and placed in a different timeline or world. That new setting can be a fantasy realm, a future world, or anything else the author deems it to be.
Badfic - a fanfic that is written badly on purpose, usually for humorous purposes.
Bashing - indicates that a character, usually specified, will be humiliated or degraded. Can be funny or romantic, or both.
Bastardization - a much stronger version of bashing. It implies that a character will be evil or unusually cruel, and is a form of OOC warning.
Canon - the official story of a series, as written or designated by the creators. If a creator clarifies something in an interview, for instance, saying that a character is indeed a lesbian, this is canon even if it was not blatantly stated in the anime or manga.
Crossover or X-over - the mixing of one, two, or more series. Characters meet, plots get mixed together, and everything becomes one.
Ecchi - comes from the Japanese pronunciation for the h in "hentai," and also means pervert. It is not quite as negative.
Fanon - facts that most fans accept as canon, but are not definitively shown within canon. Very rarely, fanon can influence canon.
Flame - rude, negative feedback. People do not have to read a fic to flame, an interesting fact, but they do have to disagree with its content. Those writers in the yaoi world are guaranteed to get this sort of thing in our inboxes: "U R SIK GO 2 HELL U MUTHAFUCKER DIS SUX!!1!!!!1!11!"
Fluff - a usually happy fic, full of lovey-dovey scenes and cute moments between characters. It can also denote a fic that is pointless.
H/C or HC - Hurt/Comfort. One character gets hurt and another character jumps in to help them. Can end up in sex or sap in some cases.
Hentai - means "pervert" in Japanese. Hentai is an NC-17 story with lemon content, or with sex that squicks many people.
Het/Hetero/Heterosexual - a fic with pairings between a male and a female. Not yaoi, not yuri.
Lemon - a graphic sex scene, or a fanfic with such a scene in it.
Lime - a story or scene that includes some kind of foreplay, such as kissing and petting, or basically anything that would lead up to the lemon, without writing out the lemon itself.
Mary Sue - the bane of fanfiction. That irritatingly perfect original character that is a more beautiful, more powerful, more desirable version of the author. She gets the character that the author wants, she saves the day - by using her spiffy powers or by dying "tragically" - and she acts like she was always a part of the universe. Most people do not like to read about Mary Sues, myself included. The male version, who is a brooding, angsty, but incredibly powerful and good-looking tough guy, is called Marty Sam, Larry Stu, Jimmy Bob, or other similar names.
MST - a parody of a fanfic, with comments in brackets between the lines of the original fanfic. MST comes from "Mystery Science Theater 3000," a show on Comedy Central and the Sci-Fi Channel. MSTs can be very funny if done properly. They are best done with the original author’s permission, or the author may face some problems.
NCS or Non-con - stands for "nonconsensual sex," which means that one party does not want to have sex. Though some people consider this the same thing as rape, NCS is often designated by token resistance, e.g., saying no but giving in at the end.
Non-canon - the fanfiction world's fuel. Anything that happens outside the official story or alters it is non-canon. Quite a lot of fanfiction fits under this definition.
Non-yaoi - indicates a fic in which there is no male/male relationship.
OOC - out of character. An OOC warning means that a character will act in a way that he or she did not/would not in the original anime or manga. Many people avoid fics with an OOC warning, myself often included, because OOC can include the attack of Mary Sues as well as ruination of canon characters.
PWP - "plot? what plot?" or "porn without plot." A fic of this type lacks plot except to arrive at the lime or lemon, or it can mean a fic that is entirely a lemon.
R&R or R/R - read and review. Authors who put this in a summary or in an author's note want to be sure that they will get feedback.
Rape - sex involving pain and obviously, a lack of consent. Goes hand in hand with angst and violence warnings most of the time.
S/M, SM, or Sadomaso - sadism/masochism. This is kind of an umbrella term in my mind, spanning from bloodplay to bondage and discipline to spanking. Sadism comes from the Marquis DeSade, who wrote about the joys of causing pain, so it means that a person gets aroused by hurting others. Masochism comes from Sacher-Masoch, who wrote about the joys of receiving pain, and means that a person gets aroused by being hurt.
Sap - a fanfic that is sweet and romantic (enough to give some readers toothaches). These are the fics that can induce their readers to romantic tears or to say, "How cute!"
Seme - the dominant one in the relationship. The one who penetrates. The one on top (or not in some cases). The screwer.
'Shipper/'Shipping - a 'shipper is someone who supports a particular pairing exclusively - 'ship stands for relationship. 'Shippers may become violent towards people who do not share their opinions. (I do not have a real 'ship, in case you were wondering.)
Shoujo ai - a story or manga that focuses on love and emotions between two or more girls, similar to shounen ai.
Shounen ai - means "boy's love," and includes stories or comics that are about two or more boys being in love or having a relationship, but it tends to focus more on emotions than sex, unlike yaoi.
Squick - a pairing or idea in a fanfic that makes a reader feel sick or that disturbs them. For example, rape squicks some people, bestiality others, and tentacle sex squicks still more people.
Troll - a fic that is written intentionally to be a joke. See badfic above.
Twincest - an incestuous relationship between twins. For example, Amiboshi and Suboshi having sex with one another is twincest.
Uke - the submissive one in the relationship. The one being penetrated. The bottom. The screwee.
Violence - physical pain in a fic, like fighting or causing abuse.
WAFF - warm and fuzzy feelings. WAFF tends to accompany sap or fluff, though it can manifest iteself in fics of any rating. It means that the fic either instills such feelings in the reader, or that such feelings are shared between characters.
Yaoi - stories or comics with male/male relationships. It is an acronym for "yama nashi, ochi nashi, imi nashi," which means, "no peak (climax), no meaning, no resolution," as yaoi tends to focus more on sex rather than emotions or plot.
Yuri - two or more females in a romantic relationship, much like yaoi. Literally translated, "yuri" means "lily."