To the unenlightened, a Mary Sue is a strange and dangerous creature who has made her (or occasionally his) way into a world created by someone else. The Mary Sue is basically the author in 'perfect human specimen' disguise (or occasionally somewhat imperfect, but secretly perfect). They somehow inveigle themselves into the main plotline, become crucial to the original pot and almost invariably fall in love with the best looking.
This might not be so bad, perhaps, if it wasn't for the fact that Mary Sues are usually, from an adult and sane point of view, not actually perfect at all. Their author thinks they are, but they are more often just rude, immature and irritating.
I have read way too many Mary Sues in the course of a misspent youth. So many, in fact, I've been driven to write one. Well. Sort of.
