Adventure:  A fictional genre involving action, danger, and unknown risks. 

Angst:  A fictional genre in which one or more of the characters suffers emotional and sometimes physical torment. 

AU:  Alternate Universe.  This term either refers specifically to the HTLJ eps, Stranger in a Stranger World and Stranger and Stranger, or fan fic set outside the show's usual timeline.

BDSM:  Bondage, Dominance, Sadism, Masochism.  Sometimes broken down into component parts. 

Comedy:  A story of light and amusing character and typically with a happy ending. 

DD:  Domestic Discipline.  Refers to a relationship between two people in which one is a disciplinarian, the other the disciplined--but not in a sexual context (if it's sexual, it's called D/s).

Drama:  A story intended to portray life or character, usually involving conflicts and emotions through action and dialogue. 

D/s:  Domination/submission.  Cf. DD.

G:  A North American film rating, indicating content (sex, violence, language) inoffensive to pratically anyone.  (See How Stuff Works for more information.)

Gen:  Variously meaning 'general', 'generic.'  Essentially refers to fic that does not depict sexual relationships, either slash or het.

Het:  Fic where the primary relationship between characters is heterosexual. May contain a relationship between a media figure and an OC.

HC:  Hurt/Comfort.  Genre where one character suffers pain, usually physical (sometimes rape), and is then tended to by another.  Sometimes leads to sex.

Incest:  Sexual relationship involving closely related characters.

Kink:  Can include any number of non-vanilla sexual practices such as BDSM, DD, D/s, scat, water sports, etc.

Mary-Sue:  An original female character who is idealized ad nauseum by her author.  Her appearance and behavior are cliched.  Sometimes used to describe OMCs, although the term there is sometimes Herkie-Sue or Marty-Sue.

M-Preg (mpreg):   Male pregnancy.   Stories with a male who becomes pregnant, with the accompanying physical changes (lactation, birth).

N/C:  Non-Consensual.  Term used for fic depicting rape or problematic sexual consent in general.

N/C issues:  Non-Consensual issues.  Lesser degree of N/C; consent may be murky, but not outright rape.

NC-17:  A North American film rating, indicating explicit sexual content (not related to NC).  (See How Stuff Works for more information.)

OFC:  Original Female Character.  This is a character invented by the author, and is often associated with Mary-Sue.

OMC:  Original Male Character.  This is a character invented by the author, and is often associated with Marty-Sue.

Ongoing:  An unfinished story.

PG:  A North American film rating, indicating content of mild violence or profanity and non-explicit sex.  (See How Stuff Works for more information.)

PWP:  Plot?  What Plot?  Stories where the emphasis is solely on sex.  Usually, they don't display much development.

R:  A North American film rating, indicating non-explicit sexual content.  (See How Stuff Works for more information.)

Romance:  A fictional genre focusing on the development of the relationship of two (occasionally more) characters; a love story. 

Scat:  Sexual play involving feces (abbr. of scatology).

Slash:  Fic playing up perceived homoerotic overtones in various cultural media. Named after the punctuation mark '/' initially used to separate Kirk/Spock, the first 'official' slash pairing.

Uber:  This term is specific to the Xenaverse, and is essentially equivalent to the AU genre.  It refers to a story set beyond the show's usual timeline.

Underage:  Sexual relationship involving a minor.

Vignette:  A short piece, usually around 3 pages, that deals more with character, etc. than plot.

Water Sports:  Sexual play involving urine.

X-Over:  Crossover.  A fic that unites characters from two shows (in this fandom, we tend to treat HTLJ and XWP as one show).