Mary Sue (sometimes abbreviated "Sue")is a term used to describe characters who are typically considered to be wish-fulfillment characters of the author. The male equivalent is known as a Gary Stu, Marty Stu, or Larry Stu. The term originates from a Star Trek fanfiction in which the eponymous fan-made character was introduced as a parody of similar stories of the time.
Mary Sues are often labeled as "perfect" characters and usually have no flaws, inconsistent flaws (the flaws vanish when they are disadvantageous to the Sue), or inconsequential flaws (like a small birthmark, or the character is rebellious but never gets punished for it.) They are often powerful, typically beautiful, and sometimes have tragic pasts for the purposes of eliciting sympathy from the audience or excusing bad behavior. Conversely, some Sues are so helpless and darling that the heroes go out of their way to save them all the time, or may be overly flawed to create a bad-girl persona. Generally, Sues are notable by the lack of true challenges to their ability.
They are most often found in fanfiction, usually for the purposes of saving the day (upstaging the original cast in the process) or for the purpose of a romantic plot with one or more of the original cast. However, characters within an original work that display similar traits may often be considered to be Mary Sues (sometimes called Canon Sues): for example, Bella Swan from the Twilight series, or Batman, are often considered to be Mary Sues.
Because of these traits, they are generally disliked in the writing community. In more recent years, there has been a backlash against use of the term itself. Critics have pointed out its overuse, particularly in regards to non-fanfiction characters and its application primarily toward female characters (despite male characters being just as overpowered if not moreso.) They also argue that the escapism and wish-fulfillment provided by these characters contribute to a healthy self-esteem, particularly in the teenage girls who often write such stories.
During Nano, writers may be concerned that their character is a Mary Sue, and multiple discussions often crop up in Character Cafe. However, since the objective of Nanowrimo is a word count and not a standard of literature, it does not preclude winning Nano.