A Trekkie's Tale was an immortalized short piece of satire written by Paula Smith in 1972, and published in Menagerie #2. The piece defined what a "Mary Sue" was in fan fiction and possibly canon works as well: a beautiful female archetype of perfection and idealization - someone who sweeps the hearts of the main characters and becomes a focus in story. A Mary Sue represents an author's (possibly a male author) idealistic vision of an author's dream girl, and allows the author to experience adventure with her through his main character.
There may also be a male version of a Mary Sue.
The story was written to mock a genre of Star Trek fan fiction that featured female characters who would save the day when Kirk and company weren't able to. It quickly centered around LieutenantMary Sue, a fifteen and a half year old Starfleet officer who was also part Vulcan.