(WiP, just like half my things. Maybe this'll be kept up-to-date.)
Ignore the mess, and the horror that awaits you. If you're looking for a specific term, ctrl+f, and if it's not on there, comment. Anyways... the terms are not in alphabetical order, and you may have to search around for them...
Drabble=A fic, generally used to express a fic of less than 500 words. Though, in technicality, the fic must be 100 words, exactly, the term is vague, and could mean anything... from 500 words and below.
One-shot=A fic, one chapter (nothing more, nothing less, hence the word "one"), that exceeds the 500 word limit that is given to the term drabble. If something is called a "one-shot" it generally is not expanded on, but that can happen.
AU=Alternative Universe. This refers to a fic with a setting not that of canon (like, for example, a Naruto fic where the characters are not ninjas.)
AH=All Human. Not that common, at least compared to AU, but it refers to the same thing, with a few differences. This term can only really be used with non-human characters (like Spyro or Edward Cullen).
Canon=Means that a certain fact, or event, has happened within the series. It means that there is no denying its validity, making it a true fact (like... Bella and Edward getting married.)
Fanon=This term, on the other hand, refers to events not mentioned within the series, and as such, is not "true", though if in an on-going series, it might become true (like... Jacob and Seth being lovers.)
Fandom=Used to describe a series, and its fanbase. This term, fandom, usually is used to refer to fan-depicted things (like art, fics, etc.)
Fanfiction=AKA fanfic or fic (as used by me). This is probably one of terms you'd already know, and as such, probably are wondering why it's mentioned. Anyways, fanfiction is used to describe a writing piece created by a fan for a series they don't own, and has no or little involvement in creating said series. Of course, the term can also be used for other fan-created items, but those will not be mentioned (until way later).
H/C=A shortened version of hurt/comfort. Basically, in the fic, a character suffers, and get comforted by either other characters, or they end up getting comfort from themself...
A/N=AKA Author's Note. This is used generally (and hopefully) in the beginning of a fic, where the author is allowed to ramble on about either their life, their love for a (fictional) character, or... just nothing...
Warnings=(Though I believe it to be something different) is used when posting something that might trigger an unpleasant experience in someone. (My thought is that it warns--tells--the reader what's in store for them without ruining the story.)
Incest=A relationship between two characters who are related. Not mutually inclusive or exclusive or shounen-ai (which we will get to later), though it might also be a shounen/shoujo-ai relationship (like I said, getting to it.) Generally done between siblings, but can also go with parent/child, or cousin/cousin.
Shounen-ai=A softer version of yaoi (which will be explained), this is used almost always in the anime/manga fandom, and means BL (Boys Love). Could be defined as slash...
Yaoi=Hard core BL. This is basically a way to point out that porn is in the fic, and that there will be boys making out with other boys. Honestly, it's the most common term in the anime fanfiction, and is (in my mind) used incorrectly. Just beware that even G rated fics use the word "yaoi".
Ratings: Depends on person, but ratings are the same. Lowest would be "G" or "K". Next would be "K+" (from ff.net). Afterwards, it'd be "PG" or "T". After that is "M" or "R". Then, if you're lucky, it's "MA" or "NC-17".
(Will continue later... when poster is more coherent...)