I got this from Tvtropes.org. The whole article is here, if your intrested in reading up on it.
The closest thing to a widely agreed-on definition is a character who has too many positive characteristics, and too few genuine flaws to be believable or interesting. Of course, despite what many tongue-in-cheek litmus tests claim, there's no objective standard for what qualifies as "too many." One thing almost everyone who uses the term agrees on is that it is generally bad. Note, that although it can be used a good way, it is almost invariably a negative.
See these articles for takes on Mary Sue that focus on certain groupings of Common Mary Sue Traits:
•Black Hole Sue — Everything is about me!
•Purity Sue — Love me!
•God Mode Sue — Power overwhelming!
•Jerk Sue — Your petty rules do not apply to me!
•Possession Sue — My favourite character is an even better version of me!
•Copy Cat Sue — I'm just like my favorite character, but even kewler!
•Relationship Sue — He's my boyfriend now!
•Sympathetic Sue — Feel sorry for me!
•Anti Sue — I'm genuinely useless, but everybody still loves me!
•Villain Sue — I have you now, my beautiful slaves! Ahahahahahahaha!
•Fixer Sue — No, that's not how it's supposed to go!
•Parody Sue — Why don't they fall for my buxom charms?
•Thirty Sue Pileup — We are Legion.