M ~ Special Agent Fox Mulder (OS Bio)
Sc ~ Special Agent Dana Scully (OS Bio)
Sk ~ Assistant Director Walter Skinner (OS Bio)
K ~ Alex Krycek (OS Bio)
D ~ Special Agent John Doggett (OS Bio)
R ~ Special Agent Monica Reyes (OS Bio)
CSM ~ The Smoking Man (OS Bio)
P ~ Pendrall
Sp ~ Jeffery Spender (OS Bio)
WMM ~ Well-Manicured Man, consortium member also known as The Brit. (OS Bio)
O ~ Other character, sometimes an original character, sometime from a different fiction venue.
 

G ~ General Audiences; anyone should be able to read the story.
PG ~ Parental Guidance; not for young children, mostly for language or mild violence.
PG-13 ~ Parental Guidance 13; not for pre-teens, heavier violence, very mild sexual situations.
R Restricted to adults; for "vanilla" type sex; things that minors shouldn't read but that won't offend most adults.
NC-17 of an adult nature; for anything that minors should not read and that might offend adults of delicate sensibilities; this goes for stories where the sex is at all graphic
BDSM and the rest of the kinky list. Put something in your intro if it's way way out there, otherwise, caveat emptor.
PWP - Plot What Plot? Often a very short story, generally with little plot or character development, in which an event, often sex, occurs just for the sake of occurring.
 

AU - Alternate Universe
H/C - Hurt and comfort
Angst - Heart-wrenching melodrama. Trauma. Worry and trouble. Very soap-operaish.
b/d - bondage and discipline. Consent implied.
BDSM - Bondage/Discipline/Sadistic/Masochist or alternately Slave/Master. BDSM is assumed to be consensual and, therefore, is not synonymous for rape or other non-consensual sexual violence.
HEA - Happy ending - story ends with characters "happy ever after"
UST - Unresolved Sexual Tension
RST - Resolved Sexual Tension
OC - Contains an original character
Schmoop - sweetly romantic
Slash - Romantic and/or sexual relationship between individuals of the same sex Ex: Sk/K Slash
XO - crossovers - a story involving more than one fictional universe. Commonly abbreviated as X-over or X/O.
Post-Ep - Story related to an episode - often used to fill in the "missing scenes" or re-write the episode as it should have been written :-)
Het - Heterosexual Sex
non-con or NC - Non-consensual sex
Rapefic - Fiction that focuses on rape and its aftermath.