Ratings

G: General audience; everyone can read.
PG: Parental guidance suggested.
PG-13: Recommended to people over age 13; parental guidance.
R: Adults only. Adult themes and situations.
NC-17: Strongly adviced only for mature audiences; over age 17. Very mature situations: violence, sex, etc.

Warnings

Slash
Fanfics in which boys get it together with boys.

Femme slash
Exact same thing as slash, but with females instead.

MPREG/Male Pregnancy
A male in the story somehow becomes pregnant. Often combined with slash.

OC/Original Character
The author presents a new character, who takes up some space in the story.

OOC/Out of Character
The character in question does not act as he/she does in their canon personality.

Spoilers
Spoilers to the series if you haven't read/seen it yet.

Violence
Graphic violence in the fic. The more violence, the higher the rating.

Language
Swearing or the like. Lots of language can make the rating go up.

Classifications

Angst
Prepare to be depressed and have a tissue box handy. Not necessarily a happy ending.

AU/Alternate Universe
Does not follow the official storyline. Most fics tend to be more or less AU, and not all of them are labelled as AU. The ones labelled AU are usually ones dealing with an alternate course in canon (for example, in Harry Potter, Lily and James were never killed).

Dark fic
Usually very angsty, can deal with death, violence, suicidal thoughts, etc. Doesn't necessarily have a happy ending.

Deathfic
One of the characters in the fic dies.

Epics
Many chaptered fics, normally the size of novels (over 50 000 words).

Fluff
Warm, fluffy pieces that leave you feeling warm and fuzzy inside. No angst.

Friendship
Plot that centers around two characters becoming friends.

Humor
Fic that's supposed to be funny, obviously...

Hurt/comfort
One of the characters gets hurt (emotionally or physically) and another character takes care of him/her.

Oneshot
One-off fics which contain the entire plot (as it exists) in a single chapter/file. Can be any genre - fluff, angst, deathfic, or other, though usually not all that much plot.

POV/Point of View
A piece told from the perspective of one character/first person POV.

Pre-Slash
Usually a fic about how two characters got together, but the fic does not deal with the said characters actually being together. Can usually be read as friendship-pieces. No kissing and such.

PWP/Plot What Plot
Meaning there isn't meant to be any deep and profound meaning behind the story. Usually refers to a fic put together simply for the benefit of getting some 'action' between the characters, often with a high rating (R or NC-17).

Romance
The romance between two (or sometimes three) characters is the main plot of the story.

Songfic
Fic with song lyrics used as a theme.