For the most part, this is THE list of words used to describe fanfics on the internet. Most sites will use this, regardless of the show being used.
Angst: Depression, torture (mental or otherwise), general bad feelings.
AU: Alternate Universe. The characters are placed in a situation impossible in the series.
Citrus/Citrusy: Yery light sexual content. Nothing more than a peck on the lips, etc. Last on the 'Citrus Scale.'
Crossover: A fic with characters from two (or more) anime series.
Death/Deathfic: Someone dies, usually as the central point of the fic.
Fluff: Light-hearted, happy stuff. No real content.
FV: Fantasy violence; Sailormoon destroying youma with Moon Tiara Action, etc.
Het/Non-Yaoi: Male/female relationships.
Humor: Self explanatory.
Language: Bad words, swearing.
Lemon: Explicit sexual content; penetration, oral sex, etc. Heaviest of the 'Citrus Scale.'
Lime: Non-excplicit sexual content; heavy kissing, petting, etc. Second on the 'Citrus Scale', just below Lemon.
NCS: Non-consentual sex, rape, beating, etc.
OOC: Out of character
OT: Out of timeline. Could have happened in the universe of the show but didn't.
Orange/Orangy: Light sexual content; light kissing, very little touching, etc. Usually last on the 'Citrus Scale.'
Original Character: Fic contains one or more original characters.
PWP: Plot? What Plot?
Sap: I love you, you love me, everybody loves everybody. Pure sugar.
Shojo/Shoujo-ai: "Girls' Love."
Shonen/Shounen-ai: "Boys' Love."
Songfic: A fic set around or to the lyrics of a song.
Squick: A fic with scenes of very brutal and/or gory violence.
Vidfic: A written video to the lyrics of a song.
V: Violence. I think you understand.
Yaoi: Male/male relationships.
Yuri: Female/female relationships.
Now that you know the general terms of fanfiction ratings, you might be wondering, "What is this Citrus Scale?" The name is just something I made up, but sexual content in fanfics is rated in terms of citrus fruits, 'Lemon' being the highest, essentially X-rated, to 'Citrus', basically Disney/G-rated. The rating can also vary depending on an author's views of the subject. Some will use 'Lemon' to describe ANY kind of sexual content at all, others will use 'Lime' to describe just about anything your mind can think up.
The other scale is the 'Yaoi' scale. Yaoi itself is an acronym, taken from the Japanese words 'Yama-nashi, ochi-nashi and imi-nashi' meaning, without climax, without conclusion and without plot. The word 'Yaoi' was created to mean a story where one male character tries to get another male character into bed. It is a shonen-ai, lemon, PWP. Shonen-ai is the lighter end, meaning stories that have or focus on male/male relationships, but with plot and not always lemony. Yuri works the same way.
I hope this clears up any questions you might have about The Internet Rating System.
