From Fan History Wiki
Mary Sue (sometimes shortened to simply Sue, or Gary Stu for a male), is a (typically) original character/fictional character with overly perfect beauty and traits, lacking all but the most minor flaws, and functioning primarily as an authorial substitute in wish-fulfillment fantasies. Typically these stories involve the Sue becoming the romantic interest of the main canon character who is the favorite of the author, or she might seem to exist only to "show up" the other characters by being able to solve mysteries, beat adversaries, and accomplish other goals the canon characters are unable to.
Mary Sue is largely a derogatory term in fandom. It is often said that the only person who likes a Mary Sue is the author who created her. However, there can be controversy over how one defines and identifies a Mary Sue. Some authors feel that many readers will reject any original character/love interest as a Mary Sue without giving the story or character a chance--and that at the same time, fans will let canon characters get away with a lot of traits and actions that should result in them being labeled Mary Sues as well. Many readers respond that they read fan-fiction for the canon characters they know, not original characters, so a story which is centered on an original character is not something they will be interested in reading and Mary Sues are easy to spot and reject.
In general, Mary Sue is seen as symptomatic of "immature" writing. Many beginning and/or young fan-fiction authors will begin writing by coming up with scenarios where an idealized version of themselves can interact with their favorite characters, hence resulting in Mary Sues. That said, many more experienced authors and readers feel that such stories should remain drawerfic -- written for and enjoyed by the author only, without subjecting others to one's personal fantasies.
The following description dates to 1868 and a collection of articles entitled "Modern Women and What is Said of Them". In an article entitled "Women's Heroines", an anonymous author described what seems to be the predecessor of the modern fan-fiction "Mary Sue":
- In manufacturing her heroines, the young recluse author puts on paper what she would herself like to be, and what she thinks she might be if only her eyes were bluer, her purse longer, or men more wise and discerning. In painting the slights offered to her favorite ideal, she conceives the slights that might possibly be offered to herself, and the triumphant way in which she would (under somewhat more auspicious circumstances) delight to live them down and trample them under foot. The vexations and the annoyances she describes with considerable spirit and accuracy. The triumph is the representation of her own delicious dreams. The grand character of the imaginary victim is but a species of phantom of her own self, taken, like the German's camel, from the depths of her own self-consciousness, and projected into cloudland.
- This is the reason why authoresses enjoy dressing up a heroine who is ill-used. They know the sensation of social martyrdom, and it is a gentle sort of revenge upon the world to publish a novel about an underrated martyr, whose merits are recognised in the end, either before or after her decease. They are probably not conscious of the precise work they are performing. They are not aware that their heroine represents what they believe they themselves would prove to be under impossible circumstances, provided they had only golden hair and a wider sphere of action.
- "Mary Sue" Stories: A story [...]that deals with the adventures of a perfect heroine, who is obviously the author as she would like to be (almost all such stories are written by women). The term derives from a story submitted to the fanzine Menagerie. The editor, Paula Smith, could not in good conscience print the story, but proceeded to write an print a parody, using the name Mary Sue for the heroine.
- Mary Sue is the youngest, smartest, most beautiful ensign on board the USS Enterprise. She outwits Klingons, seduces Mr. Spock, reinvents the Warp Drive, ad infinitum, ad nauseum. In short, Mary Sue has become the symbol of the perfect heroine, and when she appears in professional fiction, editors recognize her and insist that she be given a few flaws to make her more human, whether she is or not. (There have been a few Vulcan Mary Sue characters, too.) Mary Sue has a male counterpart "Marty Su," who is just as obnoxious. (Rogow, 196).
- Note: One of the best (and most often sited) examples of a Mary Sue in a professional work is the oft-maligned Wesley Crusher of Star Trek: The Next Generation. 'Wesley' is the late Gene Roddenberry's middle name. The main problem most people have with Mary Sues is the fact that the characters are always so jaw-achingly perfect and seem to exist only to take the spotlight away from the canonical characters of whatever fictional universe is being written about. 
- Mary Sue, n. A character who is a transparent double for the author of the story. A story featuring such a character. Although writing a Mary Sue accidentally is considered terribly gauche, many experienced authors enjoy playing with the concept in order to depict themselves in mock argument with their characters or to write their friends cameo appearances as gifts.
- The term "Mary Sue" was coined to describe original character in fanfic who are TOO perfect. These are usually heroines (hence the term 'Mary Sue'), who step in without warning or true back-story (or a very implausible or overly-used one), sweep one or more established protagonists off their feet (usually making them act completely out of character), and then proceed to save the day in truly remarkable and dramatic ways that leave the established characters gaping in admiration and amazement.
- Mary Sue:
- Type of fanfiction. Author creates a fictitious character or inserts him/herself into a videogame / movie / cartoon, affecting the developments. 
- Mary Sue: An original female character in fanfiction. Watch out—it's the ultimate OFC! Run for it! Readers have come to dread the appearance of Miss Mary Sue, the most perfect limelight-hogging heroine you'd love to hate. Mary Sue is an avatar gone wrong because she takes over the fanfic entirely, out-shining even the main players. Mary Sue is self-insertation personified, an indulgence of the author in question. But not all Mary Sues are truly bad, some can fade into the background quite well and become mature characters in their own right. More on Mary Sues here. Take a (one of many) Mary Sue Litmus test to see if you're a Mary Sue for your fandom!
- Mary Sue: A character invented by the fic author that is an overly-glorified version of the author. 
- Mary-Sue: A story in which a new character is so brilliant, witty, amazing, attractive and wonderful that one (or all) of the main canon characters completely falls head over heels for. Basically, this character tends to be a thinly disguised wannabe of the author themselves. This type of fic is usually associated with romantic stories in which the female author lives out her fantasy of being with a particular character through her story. 
- Mary Sue = A (most often female) original character inserted into the canon universe, who is really a representation of the author and everything she would want to be and do. (Read any "girl falls into Middle-earth" fics? They are usually prime examples of Mary Sues.) 
- Mary Sue - Story containing a character that closely resembles the author, or the character which closely resembles the author. 
- Mary Sue - See Self-Insert 
- Mary Sue - Somehow, I end up in the plot. Don't ask. 
- Mary-sue - a seemingly perfect character; mary-sues generally are a type of self-insertion into a fanfic, and are very annoying. 
- Mary-Sue - Any original female character which is too perfect, too extreme, or otherwise badly done. There is no real hard and fast standard for what constitutes a Mary Sue and much depends on the eye of the beholder.
- 7. fan fiction. Most people outside of the fanfic community probably think (if they think about it at all) that fan fiction is the exclusive domain of SF and fantasy fans. While this was once true, it’s certainly not true any more, and both the name and some of the associated terminology of fan fiction originated in SF fandom. Some of these associated terms are slash (fiction that depicts a sexual relationship between two characters) and Mary Sue (a character that acts out a blatant wish-fulfillment of the author or a story featuring such a character), both of which originated in Star Trek fandom. Curiously, fan fiction was originally used to refer to amateur fiction written about fans themselves, rather than amateur fiction written using the characters or settings of an existing work. 
See A historical perspective on Mary Sue for a detailed history about Mary Sue.
This section needs more information.
- on mary sue policing and why i cannot abide it by niqaeli on April 9, 2010
- more thoughts on the mary sue thing by niqaeli on April 10, 2010
- I'm no longer okay with the term "Mary Sue". by goldjadeocean on April 10, 2010
- If we get Adam Lambert to cover Peggy Sue but change the name to Mary, will that make it okay? by facetofcathy on April 10, 2010
- Such stuff as dreams are made on by goldjadeocean on April 10, 2010
- Storming the Battlements or: Why the Culture of Mary Sue Shaming is Bully Culture. by boosette on April 10, 2010
- Oh, frig, I know what I want to say; I just can't figure out how to say it. by friendshipper on April 11, 2010
- Why I love Mary Sue by miss-haitch on April 11, 2010
- My Problem With Mary Sues by thedeadparrot on April 12, 2010
- The post starts out good, about how "female characters aren't allowed to embarrass themselves". by kanata on April 12, 2010
- I have a little trouble when people tell me, "But I don't want to read a Mary Sue!" by quinfirefrorefiddle on April 13, 2010
- OK, Mary Sues have never been fandoms favourites, but is the hate coming in waves? Like every 5th year, or so? by tyffi on April 13, 2010
- There is a Mary Sue debate on by waldorph on April 13, 2010
- way too much rambling on mozart and mary sues by ar on April 13, 2010
- dissection of Mary Sue and our issues with her by tielan on April 13, 2010
- I'd been meaning to write a post called "In Defense of Mary Sue" but I see that I was beaten to it by astridv on April 13, 2010
- Write what you wanna write by cschick on April 13, 2010
- Mary Sue: Destroyer of All That Is Good and Righteous by ssquirrel-fic on April 13, 2010
- On "Mary Sue" by blythe025 on April 13, 2010
- A post full of thoughts by elvenpiratelady on April 13, 2010
- Musing about Mary Sue by tyffi on April 13, 2010
- A Very Long Entry About Mary Sues by t-eyla on April 13, 2010
- The Mary Sue Character Displacement Theory by seekergeek on April 14, 2010
- in which I am old... by vaznetti on April 14, 2010
- wherein Liz thinks about Mary Sues by edenfalling by on April 14, 2010
- Meta: Why Sues really are bad (and, yet, awesome female characters are great). by swordygardener on Apri 14, 2010
- Thoughts: I Have Too Many Male Avatars (and what that says about me) by verasteine on April 14, 2010
- Mary Sues on Metafandom by sci-frey on April 15, 2010
- What I Mean By Mary-Sue and Why I Hate Her by deird1 on April 15, 2010
- thinky thoughts on Mary (Gary) Sues by slaymesoftly on April 15, 2010
- A few (possibly contradictory) thoughts on Mary Sue. by annerbhp on April 15, 2010
- The matter is being doggedly pursued by shiegra on April 15, 2010
- Re the recent post I made about McKay and Carter as canon Mary Sues in Stargate, further notes. by cesare on April 15, 2010
- Man of Steel! by astridv on April 16, 2010
- urge to kill rising by sohotrightnow on April 16, 2010
- Further thoughts on writing fanfic by legionseagle on April 16, 2010
- She was sent from the Pit to devour us all! by bauble on April 17, 2010
- Buffy S8, canon Sues, and the impossibility of socialism under Campbell: or Protagonist Privilege by londonkds on April 17, 2010
- Dear Mary Sue Wankfest: Define Your Terms, or STFU by kimboosan on April 17, 2010
- Talkin' About Mary-Sues -- sometimes it isn't about writing by googlebrat on April 17, 2010
- In which I ramble about Mary Sues by nevacaruso on April 18, 2010
- The Five Faces of Mary Sue by sci-frey on April 18, 2010