*Irony, thy name is Blue*
A Mary-Sue (or a Gary-Stu, for a male) is the character who is 'special'...
so VERY special that there are several hundred thousand of them in modern literature. *Gag*
Usually, she is ALWAYS right, has eyes/hair that change colours, a magic power of some sort, and everyone loves her. She sometimes sacrifices herself to save others, but she usually survives by some strange means. She has a weird name. She can be a total witch, and everyone still wants to be her best friend. She is loved by the hottest guy in the story.