can you tell me what these abbreviation type things mean or stand for?

AU: Alternate Universe. If you put the characters you're a fan of in a different universe.
For example: make Angel 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. Writing 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 love with her... and ofcourse she will defeat Voldemort while looking like an angel.
There are different kind of Sues, but you can recognize them easily: reading 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 by animals could calm animals 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 author or reader. Perhaps the single underlying feature of all characters described as "Mary Sues" is that they are too ostentatious for the audience's taste, or that the author seems to favor the character too highly. The author may seem to push how exceptional and wonderful the "Mary Sue" character is on his or her audience, sometimes leading the audience to dislike or 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" by readers are not intended by authors as such. Male Mary Sues are often dubbed "Gary Stu", "Larry Stu", "Marty Stu", or similar names.

While the term is generally limited to fan-created characters, and its most common usage today occurs within the fan fiction community or in reference to fan fiction, original characters in role-playing games or literary canon are also sometimes criticized as being "Mary Sues" or "canon Sues," if they dominate the spotlight or are too unrealistic or 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 - you know, they have it all and so on and so on. like Bella Swan for example.