User blog comment:LemonFairy/On Writing Well (not by William Zinsser) - I/@comment-1897744-20100312115123

A Mary Sue is a character that is perfect, or near perfect. Her character often lacks depth, and the only flaws she has are superficial ones that would not hurt her in a place where it truly matters. She often has an angsty, tragic past and many times is either the wish fulfillment of the author (self insert) or the child of two canon characters - the more unlikely to get together, or the more canon defying, the better.

For a good example of a published Sue see Paolini's Arya or Meyers' Bella Swan. The male equivalent has been named many things, but "Gary Stu" is most often adopted.

http://en.wikipedia.org/wiki/Mary_Sue

See the article above for a full definition and explanation of Mary Sues.