Just so all the little newbies out there don't get lost in any of these rants…. Most of these things should be very duh-ish, but you'd be surprised what some folks don't know, so I'm covering all of my bases.
Alt. Reality: Alternative Reality. A fic with canon characters, but the details are all off. I.E. A fic where the Moon Kingdom was never actually destroyed. Also known as an Alternate Universe or AU fic.
ANs: Authors Notes. Located usually in the beginning or end of the fic, it's where the author of a fic thanks all of those that need thanking, explains whatever needs explaining and is where the disclaimer is located.
Archive: A website that houses many fanfics. In the Sailor Moon fandom, the dominant archive is A Sailor Moon Romance.
Author Avatar: When the author inserts him/herself into his/her fanfic as a character.
Canon: The official continuity of a show/book/movie as developed by its creator. All fanfics exist outside of canon though they might reference canon events.
Chaosfic: A fanfic written by his Lordship Chaos while he is in either his real or avatar form. All Chaosfics written by the avatar don't actually exist and are, instead, running jokes for his "Curse of the Fanboys" series.
Disclaimer: A fun little statement saying, "Hi! I don't own this! Please don't sue me!"
Fanfic: Short for fanfiction. A work of fiction written about a set of pre-existing and trademarked characters – usually from a TV show or movie – written by fans for non-profit/entertainment purposes.
FFN: Fanfiction.net. A HUGE archive of every genre. Has the highest crap per fic ratio of any archive in any genre EVER.
Hentai: A fanfic porno.
Lemon: See "hentai".
Lime: Not quite a lemon. A lime, unlike its lemon counterpart, doesn't actually describe any implicit sex. The sex is implied, but one never reads it. You might read some of the foreplay, and a little of what happened AFTER the act, but never the sex itself.
Mary-Sue: The most insidious of all author avatars. Mary-Sue is the most perfect individual ever. Everyone likes her. She has no flaws. She gets to screw the author's favorite character (or at least be related to her). She looks like a model, has all of the good lines, and kicks the most ass. Where, oh where, would our CANON characters ever be without dear little Mary? The original Mary-Sue was a character (named "Mary-Sue") in a Star Trek: The Next Generation fanfic.
Marty-Stu: The male version of Mary-Sue. Also known as "Marty-Lou" and "Montgomery".
MiST: Any fic that has received the "Mystery Science Theater 3000" (i.e. MST3K) treatment. It can, but does not need to use the set characters and format of the show. Also known as a MST in Moonie-dom.
MiSTing: The act of writing a MiST.
MiSTie: If a fan of MST3K. Also, if a fic is a MiST, then one says that it has been MiSTied.
MST3K: Mystery Science Theater 3000. A show that aired formerly on a private cable station for two seasons, Comedy Central for five seasons, and two seasons on Sci-Fi. It featured some hapless man from Earth (first Joel and then Mike) who was captured and put on the "Satellite of Love" where he and these two robots were forced to watch bad movies as part of a scientific experiment. The only way they could handle the bad films was to openly mock the movie.
Moonie: What you call a Sailor Moon fan.
Oscar: The name of the worst author in SM. He wrote, among other things, "Artemis's Lover". No intact forms of his fics still exist, but the MiSTs by Megane 6.7 still live.
Slash: A homo-erotic hentai. It initially was a term used to describe any non-standard pairing of characters (I.E. a Usagi/Demando fic) but eventually morphed into focusing solely on same-gender relationships.
SM: An abbreviation for Sailor Moon.
Yuri: An all-female slash. The most common form of Slash found in Sailor Moon.
Yaoi: An all-male slash.