A poor character, usually of fanfiction, and often a new addition by the author to the fandom. Sues are always female; their male counterpart is a Stu. Sues are self-insert characters more often than not. That is, they are author surrogate characters that represent how the author might design herself if she was in control of the world.
In particular, Sues often have perfect traits on the edge of unrealism: unfailing beauty, exceptional charisma, sustained strength and/or ability. In addition, Sues also have boo-hoo tragic pasts, including but not limited to: entire family being murdered at a young age, being disrespected and/or unwanted for a majority of early life, slavery, etc etc etc. Yet somehow, they are able to function well and pull through the misery.
In conclusion, Sues are horribly written, horribly read self-insert characters with unrealistic traits, even for fanfiction. Although some Sues can be well-written and therefore pleasing, most suck. Never read one and certainly never write one. For the male counterpart, see Stu
Xylanding'am Sue, a stunningly beautiful young girl has had her entire family murdered when she was a child and she barely escaped with her life, she now goes on a dangerous quest to find a fifteen long foot sword and defeat the evil tyrant rulers blah blah blah...
Harry Potter's parents have been murdered at birth giving him a physically impossible scar on his majestic forehead, despite rejection at an early age he has managed to become a big hero.
Bella Swan has...oh, don't even make me MENTION her.