Mary Sue was a nice young girl, dancing around in the world of Star Trek fanfiction with her boyfriend Gary Stu, but slowly she started drifting out of the world of fanfiction and into author pages.
So who is Mary Sue? A Mary Sue character is something you want to avoid as a writer. They are characters who are just too perfect with everything turning up roses for them. She has the Midas touch, gets all the guys, does nothing wrong and is the author's pet. And readers spot her a mile away and go running for the hills.
Nowadays a Mary Sue/Gary Stu can also include a character that acts as an author surrogate - a character that inflicts the creators personal beliefs onto the reader - or a cliched character in desperate need of a make over.
It doesn't matter if you are writing about aliens, paranormal creatures or 'regular folk', characters need to have flaws. If a character overcomes every obstacle put before them with ease, how is that entertaining the reader? Ho hum, he beat that troll with his his bare hands - boring. Oh now she has five guys chasing after her. She's so beautiful, but doesn't know it. Gah! How are we meant to like this chick?
People want variety, they want originality. They don't want to have a stock standard girl/guy pop up in every book they read. It's important to not follow 'trends' for your heroes or villains. It can be hard as there is only a set number of hair colours and eye colours to choose from, however all the best friends' characters don't have to have red hair. Nor does the MC have to have green eyes.
Make your characters original, make them pop from the pages for your writers. Give them someone new and refreshing to fall in love with.
If you want to know if you have written a Mary Sue/Gary Stu in your story try this test (I passed!!):
More info on Mary Sues and how to avoid them: