A female fanfiction character who is so perfect as to be annoying. The male equivlalent is the Marty-Stu.
Often abbreviated to "Sue". A Mary Sue character is usually written by a
beginning author. Often, the Mary Sue is a self-insert with a few
"improvements" (ex. better body, more popular, etc). The Mary Sue
character is almost always beautiful, smart, etc... In short, she is the
"perfect" girl. The Mary Sue usually falls in love with the author's
favorite character(s) and winds up upstaging all of the other characters
in the book/series/universe. There are several main types of Mary Sue:
Victim!Sues: The Victim!Sue is your whiny, wimpy, pathetic female character who can't seem to do much of anything except cry and get herself into trouble that the romantic interest of the fic has to rescue her from.
Warrior!Sues: The Warrior!Sue is usually loud, obnoxious and (of course) an amazing warrior. She'll usually have some tragic past that led her to become a warrior, and she'll upstage all of the Canonical characters with her mad Sueish powerz.
Mage!Sue: Similar to the Warrior!Sue, the Mage!Sue has amazing stregnth in magic, or has a magical power that nobody else has. She'll usually wind up upstaging all of the magical characters of the series.
Punk!Sue: Also called Noncomformist!Sue or Goth!Sue, the Punk!Sue is usually written by female beginners in the 11-15 age group. The
Punk!Sue is loud, obnoxious, annoying and generally the type of person
who you'd want to send off to boot camp for six months. The Punk!Sue
almost always has angst coming out of her ears and isn't really a bad
person, she's just oh!-so-angry at whatever tragic past the author has
chosen to give her. The Punk!Sue is based on what the 11-15 year old
author thinks is "cool" and wishes she could be. This includes
Misfit!Sue: This includes all Sues who are supposedly geeks, nerds, misfits, etc. Usually, the Misfit!Sue doesn't start out as inhumanly beautiful, but winds up getting a makeover and finding out she had the potential to be a guy-magnet (or girl-magnet, depending on the genre) all along. Also includes the "My parents want me to do this but I want to do that and it's not fair!!one!" type of Sue. Usually, this Sue is very bookish and smart, but will find some sort of physical talent nobody expected and become a star as a result.
Another thing to note is that a Sue will usually have a completely off-the-wall name, like "Viquetoria". The more wierd and pretentious the name of the character, the more likely it is that she's a Sue.
Finally, Sues often have wierd, improbable or impossible bloodlines. A secret half-elf child of Elrond and a nameless human would be an example of this. A character who was Dumbledore's grandchild and Tom Riddle's daughter would be another example.