Here are some common fanfiction terms and abbreviations that I feel you should probably know. In alphabetical order, no less!
"AU" - Alternate Universe. AUs are different versions of Les Miz, for instance, Les Miz if Jean Valjean was never redeemed. If you choose to write an AU, make sure you make it very clear that it is AU.
"Canon"- The original, truest form of a fandom. For Les Miz, canon is the novel.
"Drabble"- A story told in exactly 100 words. I had a little trouble with this one!
"Femmeslash"- A woman/woman relationship. Les Miz femmeslash is not exactly possible. I guess you could go Matelote/Gibelotte (novel only characters) but I wouldn't recommend it...
"Ficlet"- A one chapter story or a short fanfiction that is over or under 100 words.
"Flame" Almost everyone know this, but it's a especially mean review, usually to the effect of "omg yur stori sux" If someone reviews with an outline of what you did wrong and how to fix it, that isn't a flame, that's a helpful review.
"Mary-Sue"- A female OC who is perfect. Quite possibly the number one fanfiction mistakes. Mary-Sues are quite hated.
"OC"- Original Character.
"One-shot"- A one chapter story.
"OOC"- Out of character. A breakdancing Jehan is certainly OOC.
"Slash"- Believe it or not, it took me months to figure this out. Slash is a man/man relationship.