AU - Alternate Universe, basically any fanfiction that has altered by the author.  Applies to any changes such as additional main characters or new locations.  Usually only used to describe the latter of the two.

Hentai - {literal translation-pervert(ed)} used to describe fics and stories featuring mature sexual themes

Het - short for heterosexual.  Fics featuring male/female pairings

Lemon - fics featuring sex

Lime - fics hinting at sex, like in an R-rated movie

Mary Sue - one of two character types: either a self-insert or a character with no flaws, more power than all the Z senshi, and all the characters immediately trust him/her.  Or one or more of the original characters falls madly in love with him/her, forsaking their husband/wife.

OOC - Out of Character, having one of the canon characters act completely unlike they would ever act

OC - Original Character, a character you create

PWP? - Plot What Plot?, used to describe fics written solely for sex

Shojo-ai - female/female romance stories

Shonen-ai - male/male romance stories

TWT - Time What Time?, stories that completely disregard the original timeline

Yaoi - male/male lemon stories

Yuri - female/female lemon stories