can Du tell me what these abbreviation type things mean oder stand for?

AU: Alternate Universe. If Du put the characters you're a Fan of in a different universe.
For example: make Angel – Jäger der Finsternis and Spike human and go to high school with Buffy, who is not the Slayer.
Or: Make the characters from Gossip Girl vampires.

OC: Original Characters. Schreiben a new character to introduce in an existing universe.
For example: write a new student into the Harry Potter universe.

Mary Sue: Mostly Mary Sue's are OC's that are perfect. In my Harry Potter example, the new student, Mary Sue, would be good at every class and quidditch, everyone would like her (from all houses), every boy will fall in Liebe with her... and ofcourse she will defeat Voldemort while looking like an angel.
There are different kind of Sues, but Du can recognize them easily: Lesen a story with a Mary Sue is f*cking boring.

well i had one of my RPG charictors called a mary sue once...then a slight fight started...-_-" i guess it means making a charictor seem perfect then trying to push it on everyone else...i thought i was reasonable to think the a charictor raised Von Tiere could calm Tiere down, but apparently not to them. this is word 4 word what they said.
A Mary Sue (sometimes just Sue), in fanfiction, is a fictional character with overly idealized and hackneyed mannerisms, lacking noteworthy flaws, and primarily functioning as a wish-fulfillment Fantasy for the Autor oder reader. Perhaps the single underlying feature of all characters described as "Mary Sues" is that they are too ostentatious for the audience's taste, oder that the Autor seems to favor the character too highly. The Autor may seem to push how exceptional and wonderful the "Mary Sue" character is on his oder her audience, sometimes leading the audience to dislike oder even resent the character fairly quickly; such a character could be described as an "author's pet".

While the label "Mary Sue" itself originates from a parody of this type of character, most characters labeled "Mary Sues" Von readers are not intended Von authors as such. Male Mary Sues are often dubbed "Gary Stu", "Larry Stu", "Marty Stu", oder similar names.

While the term is generally limited to fan-created characters, and its most common usage today occurs within the Fan fiction community oder in reference to Fan fiction, original characters in role-playing games oder literary canon are also sometimes criticized as being "Mary Sues" oder "canon Sues," if they dominate the spotlight oder are too unrealistic oder unlikely in other ways.)

AU - alternate universe. basics if something is non-canon (I'm pretty sure), like OC sisters/brothers, not following timelines and such.
OC - other character
Mary Sues are basically characters who are too perfect and cliché - Du know, they have it all and so on and so on. like Bella schwan for example.