Wikipedia: A Mary Sue, in fanfiction, is a fictional character with overly idealized and hackneyed mannerisms, lacking noteworthy flaws, and primarily functioning as a wish-fulfillment fantasy for the author or reader. Perhaps the single underlying feature of all characters described as "Mary Sues" is that they are too ostentatious for the audience's taste, or that the author seems to favor the character too highly. The author may seem to push how exceptional and wonderful the "Mary Sue" character is on his or her audience, sometimes leading the audience to dislike or even resent the character fairly quickly; such a character could be described as an "author's pet".

These are the meanings of a couple of the words that may seem confusing:
hackneyed-cliche
ostentatious-boasting

Mary Sue is recognizable by being amazigly beautiful,althought she is modest and deny it, everybody else tend to think she is an angel and fells inlove with her. The smal group of people who doesn't, change their mind later on or is a bitch or a villain.
She is smart and masters every skill, she doesn't seem to have a problem with anything. (Example:NORMAL people need years to earn the title of grat dancers, but she is "natural" and becomes best in class in matter of days.)
Thats leads to next point: She gets anything she wants without any hard work. Popularity, love of her life, dream job, cute kids and judo championship cup, Mary Sue has it all. (And kittens ADORE her!)
She is the chosen one, the only one that can save city/Earth/the Universe.

How Mary Sue is created?
Author comes to close to character and often let the character do everything he/she would like to, close phisical resemblance between them is not an exeption. Name of Mary Sue often means "beautiful" or is somehow else conected to her aperence. Mary Sue is author`s self-insert or better-self.

One or two things from the list does not necessarily make character a Mary Sue, but Bella has unusualy lot Mary Sue characteristics.
Exeptionaly, there ARE some good Mary Sues, but only because belivable plot and background story is provided, that explains her Mary-Suesnes. (The Bride from Kill Bill)

Male alternative to Mary Sue is Gary Stu.

OMG that is the exact definition of bella!

Mary-Sue: The bane of fanfiction and book readers everywhere.