What's a fanfic?
Fanfic is works written by fans on based on their favorite pairings. Some writers create their own characters (OC) to interact with the canon characters. Occasionally, some female writers get too obsessed with one particular character. In this case, she creates a character that she wishes is herself. This type of character is called Mary Sue.
What's a Mary Sue?
Mary Sue is the embodiment of the writer's fantasy character. She is generally flawless, talented, pretty, irresistible Nearly everyone wants to have a go with her. Mary Sues differ from Original Characters because they are mostly disliked by other readers. They are also the central character the story spins around. The canon characters are just there to satisfy her superiority. Mary Sues are so perfect that they become extremely annoying. People read fanfic because they want to fantasize their favorite pairing... not about some bimbo hottie getting the dude. Of course, the Mary Sue writers don't see it that way. To them, their Mary Sue is perfect. Perfect for the male canon character the author desires (in another word, Mary Sue = author themselves sexing up the male).
You can probably tell from my tone that I loathe Mary Sues... I avoid them like plague. I can rant on and on until I bore you all away, but I'm not going to do that. I'm going to show you an amazing parody song... No one can explains the definition of Mary Sue better than this.
But I have to show you one perfect example of Mary Sue... Even a noob reader can tell it's her! (I'm still debating whether the writer is a troll or a really, really, really desperate girl) Behold the infamous My Immortal that most Harry Potter fanfic readers know about... if you don't know about it... now you do! (Ps. I feel mean laughing at it but as a normal human being... it's irresistible!)
"WHAT THE HELL ARE YOU DOING YOU MOTHERFUKERS!"