Navigate over here! Each section has more links that will also appear below.
Went through and edited things as of June 17, 2007.
The most applicable for my stories: Slash / Fem Slash / Het / and Gen -- Slash is man on man, Fem Slash is girl on girl, Het is guy on girl/girl on guy, and Gen is the "just friends" area.
AU-- "Alternate Universe", meaning not in the same plotline/timeline as the original fandom. This can simply mean that perhaps in Final Fantasy VII, Zack never died and was in the game all along. It can also mean that the cast of characters from Buffy are suddenly transported to play noblemen in an 18th century French court.
TWT -- "Timeline, what timeline?" I've seen every once and a while. It means that the timeline is pretty much disregarded for the fandom although the original plot is still lurking around.
Fusion/Xovers -- Okay, fusions, from what I have always interpreted, mean that two (or more) fandoms are taken and one set of characters replaces the other set. Example - Stargate characters play the roles of the Star Wars cast. Crossovers (X-overs) refer to two or more fandoms coming together in one universe or another and characters from both series are still there. Like... Scooby-doo meets Batman.
PWP -- "Plot? What Plot?" Typically used as a heading for fics focused solely on a sex-driven plot. However, this is not always the case. It could just be a random ficlet that simply doesn't have much plot. Like in very short drabbles.
OOC -- Ah, applaude the authors who will acknowledge this heading, because it typically means they've done it intentionally and have reasons behind it. OOC stands for "out-of-character" meaning that a character isn't acting as he/she/it normally would. For example, Tifa's in love with Sephiroth.
OC -- OC stands for "other character" and is usually used in headings when pairings are stated. I.e.: Seiji + OC / Seiji x OC. The OC is a character created by the author, an original character, and may or may not be a mary sue. Also with this are OMC and OFC--"other male character" and "other female character" respectively.
BDSM -- Let's see if I can remember the proper thing it translates too... I'm not positive it's right, but it's along the lines of: "bondage-dominance-sadism-masochism". Obviously sex-related. If you don't know what it is, heed the warning and look for something else.
Lemon -- Ah, lemons, the "code-name" for sex scenes in the realm of anime/manga genres. A lemon means that there's a graphic description of sex in the fic.
Lime -- Limes are a lesser degree of lemons. There's a lot of interpretations of this, just as there are for lemons. The two ways you can look at this are: a) a scene leading up to a lemon but then is cut short before becoming very descriptive, or b) a sex scene with no real details of the act.
Angst -- I'm sure we've all heard of teen angst. Anyway, angst refers to grief-ridden, sorrow-striken, horribly sad scenes and emotions. Okay, so it's not always so extremee. If the warning's there you probably want to heed it if you don't like angst.
Shonen-ai -- literally "boys' love" it refers to a more emotionally-focused or yet-to-be physcially-developed relationship between two men/boys. Anime/manga realm. Usually portrayed in a name + name fashion. (Example, Touma + Seiji)
Yaoi -- Yaoi, when it comes down to it, is "slash" in the anime/manga world. Yaoi is a more physical relationship between two men. Meaning sexual, if you didn't catch my drift. Usually portrayed in name x name fashion. (Exmaple, Treize x Zechs)
Shoujo-ai -- literally "girl love" it refers to a more emotionally-focused or yet-to-be physcially-developed relationship between two women/girls. Usually portrayed in a name + name fashion. (Example, Rikku + Yuna)
Yuri -- Yuri is a more physical relationship than shoujo-ai between two women. The "femslash" of anime/manga. Again, sexual. Usually portrayed in name x name fashion. (Example, Haruka x Michiru)
Het -- Short for "heterosexual" pairing. Any coupling of a male or female--across all fic genres. Again, portrayed name + name, or name x name. (Exmaples, Duo + Relena, Jack x Daniel)
The "+" and "x" and "/" symbols -- What the heck are + x / ?? You'll see all three when pairings are noted in fanfiction headings (most applicable to anime/manga realms). The name coming before + or x is the dominant one in the relationship ("seme" in yaoi) and the name after is the submissive partner ("uke" in yaoi). I use the "/", as do others, to denote that neither partner is the "assigned" dominant or submissive. Examples: Touma x Seiji would mean Touma is the one "penetrating" and Seiji is the one being "penetrated". (That sound so crass, I know -_-) Shion+KOS-MOS would just mean Shion takes the lead and initiative in the relationship and KOS-MOS is more following along, although is an active participant, isn't usually the one to initiate things.Treize/Zechs is my universal example of the "/" symbol because I can never see either totally relinquishing dominance. Merely means both will initiate things and in the case of sex, the roles of top and bottomcan be switched.