I have never written nor read any fan fiction (at least not without irony). There’s nothing wrong with it, but it ain’t my bag. I was, however, intrigued to learn about the common fan fiction archetype Mary Sue. From the esteemed (and very postmodern) Official Mary Sue Society Avatar Appreciation Site:
She (or he) is created to serve one purpose: wish fulfilment. When a writer invents someone through whom he/she can have fantastic adventures, do exciting things, and meet famous people (usually but not always fictional), this character is a Mary Sue.
Although storytellers have been creating Mary Sue over and over since the dawn of time, she did not receive her current name until the early 1970s. The original was Lieutenant Mary Sue (“the youngest Lieutenant in the fleet — only fifteen and a half years old”) as immortalized in Paula Smith’s “A Trekkie’s Tale,” which she wrote and published in her 1974 fanzine Menagerie #2.
Fantastic. I love jargon with a storied etymology. Perhaps in 50 years Mary Sue will be up there with mondegreen or spoonerism (Todd, the linguistics major recently back from France, taught me that one a few years ago). Here are a couple other pages on Mary Sue.