yaoi or slash: male/male relationships.

het: heterosexual relationships.

yuri or femslash: female/female relationships.

gen: no relationships, or only vaguely alluded to relationships.

lemon: explicit and/or detailed sexual activity.

lime: implied or subtley described sexual activity.

[Apart from these two I may label some fics 'citrus' if I'm not entirely certain where along the sexual continuum they rank. :p]

dark: exactly what it says, these stories deal with darker stuff so they'e not always of the faint-hearted.

romance: exactly what it says, deals with romantic storylines.

AU: stands for Alternate Universe, meaning I've taken characters from their original setting and written them into a different world, or timeline, or changed significant canon details.

I reserve the right to add more or fine-tune definitions if I feel like it.