Just a few common fan fiction terms:

Angst: Emotional pain. Also fanfic in which a character’s emotional pain is a central plot element. Essential in my fan fiction.
au or a/u: Alternate universe. Fanfic in which the story universe is radically altered from the canon show universe. Some of my fics are set in AU:s.
B/D, B/D, BD: Bondage and domination.
BDSM: The story contains both b&d and s&m.
Crossover or x-over: Fanfic using characters/scenarios from two or more different shows. There are some crossover fics on this site.
DAL: Deep abiding love.
f: Any show, non-major-character female.
h: Humor.
H/C: Hurt/Comfort. I use this quite a lot too. At least to me, angst and h/c go hand in hand.
m : Any show, non-major-character male.
m/m: male/male sex (or romance)
Mary Sue: A character who is a transparent double for the author of the story. A story featuring such a character. A male Mary Sue is a Marty Stu. (Wesley Crusher in Star Trek TNG is a typical example). Although original characters don’t necessarily have to be Mary Sues, they tend not to be very popular in fan fiction.
MSR: Mulder – Scully romance.
m/m/f, f/f/f, m/m/m/m: Multiple or multi. Multiple partners.
NC: Non-consensual sex.
PWP: Plot, what plot? A sex story, pure and simple.
R: Romance.
R: Rape story. Fanfic in which a character is raped. Usually implies an H/C plot.
Ratings: You’ll find the list of ratings here.
RST: Resolved sexual tension.
s & m, s/m, sm: Sadism and masochism. Always consensual.
S: Slash. A story containing same sex pairings. Usually very explicit, but doesn’t have to be.
UST: Unresolved sexual tension.
V: Vignette.