Alternate Universe (AU) Fanfiction

 What is Alternate Universe (AU) Fanfiction?
    I really like the definition found on Wikipedia:
An alternative universe or alternate universe (AU) story is one that makes major changes to the canonical storyline or premise, such as killing off the main character, changing characters' motives or alliances or, most commonly, changing the setting. Generally, to be considered an alternative universe story, the change must be something that would be extremely unlikely to happen in canon, or must be contradicted by new canon information that was not released when the story was first written.
    More info @ Wikipedia.