"Mary Sue" is a degratory term for a "perfect" character, one who implausibly has no flaws or is otherwise idealized. She/he may be excessively beautiful and be the center of attention. They may have powers that don't fit the universe. They might be the long-lost sibling of the main character. They may even be their creator themselves! (i.e, Chris-Chan in the Sonichu comics)
Basically, the stereotypical "original" character you might see in fan work. But a Sue may show up outside of this media.
Here's an article that goes into the details, if you want to delve deeper.
http://tvtropes.org/pmwiki/pmwiki.php/Main/MarySue