Pairings in any given fic are indicated as Character AxCharacter B. For threesomes/moresomes, the format is Character AxCharacter BxCharacter C and so forth.

Settings-
WH40K- Warhammer 40,000
WHFB- Warhammer Fantasy Battle
MtG- Magic: the Gathering
WoD- World of Darkness ("n" or "o" at the beginning signifies whether it's New World of Darkness or Old World of Darkness, respectively)
DnD- Dungeons and Dragons
Fantasy (for non-specific and original fantasy settings)
Sci-Fi (for non-specific and original science fiction settings)
TES- The Elder Scrolls
L5R- Legend of the Five Rings
EP- Eclipse Phase
CoC- Call of Cthulhu
Monstergirl (for nonspecific settings where monstergirls are a prominent feature)
Original- setting is original and doesn't fit into any other category

Fetishes
M/M- Gay sex is present.(F/F is lesbianism.)
Anal- Anal sex is present.
Oral- Oral sex is present. (Add :female for cunnilingus, and :male for fellatio, aka blowjobs.)
Vaginal- Vaginal sex is present.
Tittyfuck- The penis is inserted in between the breasts. Usually happens alongside a blowjob.
/ss/- Short for "Straight Shota". Typically involves a young (early to mid-teens) male and a comparatively older female.
BDSM- The use of restraints and/or clearly defined dominant and submissive roles. In some cases, the infliction or receiving of pain may also occur.
Rape- Forced, nonconsensual sexual activity of some form (usually carried out via physical force or coercion). "Reverse rape" refers to cases in which a male is raped by a female. Gang-rape is a specific form of this which occurs when a single person is raped by multiple individuals. 
Incest-  Sexual activity occurring between blood relatives.
TF- The transformation (willing or not) of one character into someone/something with dramatically different physical features, which may be pleasurable. Also known as "genderbender" or "TG" if the participant's gender is the only thing that is changed.
Orgasm denial- Prolonged maintanance of sexual arousal without orgasm.
Voyeurism- Surreptitious observation of sexual activity by someone not directly involved in it.
Exhibitionism- Public nudity or sexual activity performed in a public area (or any place where it is likely to be observed).
Femdom- A variation of BDSM in which the woman is in a dominant role. In practice this is often interchangeable with role reversal, although technically they're different. 
Gore- Extreme violence and/or dismemberment occurring within a sexual context. "Snuff" is a subtype of this where one of the participants is killed.
DP- Short for "double penetration". A character (typically female) is penetrated in two orifices at once. Usually occurs in association with group sex or tentacles.
Tentacles- Penetration occurs via something with tentacles. As a rule, the sexual activity involved is usually rape of some form (most often due to non-sentience on the part of the tentacled entity).
Bestiality- Sexual activity occurring between a human and a non-sentient animal. 
Futa/Futanari- A female character who possesses male genitals (read: a dick).
Pegging- penetrating a male's anus via a strap-on or sex toy.
Unbirthing- the act in which a character is consumed by another character's vagina.
Mind control- a character is hypnotized or is under the control of another.
Facesitting- sitting on a person's face, usually to facilitate oral sex.
Sex slavery- a character is forced into servitude for sexual purposes. Unlike BDSM, this is not voluntary. 
Footplay- Sexual activity focusing on the feet.
Vanilla: Sexual activity with no particularly unusual features. Also known as "heterosexual sex in the missionary position" and often associated with feelings of mutual love, trust, and affection. /d/ finds it horrifying. 
Dubcon: Short for "dubious consent". Not rape per se, but not truly consensual either.
Knifeplay- The use of knives in a sexual context.
Earplay- Sexual activity with emphasis on the ears (especially common when elves and Eldar are involved).
Clothing destruction- violent removal or ripping of clothing.
Impregnation- a female character is made pregnant as a direct result of sexual activity within the story.
Pregnant- Sexual activity occurs where a character (usually a female) is pregnant at the time it occurs.
Yaoi- an alternate term for male/male relationships.
Yuri- an alternate term for female/female relationships.
Loli- short for "lolicon". The story features a female character who appears very young (but not necessarily underage).
No smut- Self-explanatory. That doesn't mean it's not worth reading anyway, of course.
NTR- short for "netorare", a Japanese term that describes cheating with an emphasis on betrayal and humiliation of the individual being cheated on. Strongly disliked by many people due to its emotional effects.
Watersports: a euphemism for urination-related fetishes.
Role Reversal: The male and female relationship roles are inverted; in practice, this is similar to femdom but with no element of dominance or submission.