I write fan fiction to expand upon the world that has already been created. As in, I write what should have happened in addition to the existing events, but didn't.

Like, well... the only one I've got so far is the Chrono Trigger one I'm working on, and let's face it - awesome as that game is, it really only gives the skeletons of the characters' personality. If I wanted to write my own story, I'm sure I could - it's just that before that, I want to get my own perspective on what these existing characters were thinking and feeling out of my own head and onto paper.

It's also good if you want to play around in a specific set of cirumstances. Sure, I could write a perfectly good original story, but if I wanted to write a story about, say, a girl who talks to God and uses His commands to live her daily life, I can write a Joan of Arcadia fanfic, or I can write an original story and constantly be accused of ripping off JoA.

ok since I actually took the time to read this, I have to at this point to stand up for one of my former favorite pastimes. It is very true that many, many, many pieces of fanfiction are poorly written, poorly planned, plotless, grammarless, punctuation-challenged Mary-Sues and all around shoddy pieces of crap with the orignial work taken into little or no consideration.

However, the good ones can take the original work to a whole new level. A while back on the hp spoiler thread Lady Blanc posted a link for a fic called "After the End" which was so fantastic and fit so well with jk's world that if I never read the 7th book, I would still be happy. It's no so much about being published, as much as saying these are my favorite characters in a great series/book/whatever, I wonder what would happen if this happened to them. A good writer will show you exactly how they percieve the characters and draw you in further to the world than the original. Of course some of it is just great porn that would never, ever be written into the original story, but that we all want to see anyway. ^_^

Now right here I do have to mention that I really haven't read anything book-based so that's a whole different kettle of fish that I won't get into. But when you do anime based stuff a lot of good fics are POV where you get into the character's head and show thoughts and motivations on a level that they really can't quite do on screen.

And another thing, just changing names and stuff will not just suddenly make something original. Sure if it's an AU that might work, but a lot of stuff is as dependant on the world and plot of the original work as it is on the characters. That is not to say that there aren't many fics that could be changed a bit and published as original works without stepping on any toes or appearing as rip-offs, because I've seen good ones out there where the writer did just that ("Dancing with Rude Boys" comes to mind). However, I think it's much more about having fun with your favorite characters and then letting others see what new path your mind wandered down because chances are if you enjoyed writing it, they will enjoy reading it...

Aw come on, just let us have our pretty boys with their explosive toys and let us put them through hell, strange dimensions, the occasional piece of sappy fluff, tear their minds apart with guilt and insanity, and then top it off with making them sleep together alright?
:::coughs:: yeah I'm so a Gundam Wing fan....

As a writer myself, I really dislike fanfic. I've read a lot of it --including as much as I could stand to of "After the End" --and never found any of it readable. Generally people use pre-established characters out of a certain laziness --a desire to use the shorthand of "we all know this person."

The only thing I dislike worse is using real people as characters in your fiction, which admittedly, many legitimate authors do. But it seems so disrespectful.

To me it all goes in the same catagory as "sequels by other authors," and using footage of dead people in commericals.

On the other hand, I'm not at all opposed to stealing liberally from other authors, or to presenting fictionalized versions of real people in your writing. Just give them (your derivative characters) new names and let them be their own people.

I understand fanfic as a stopping place on the journey towards being a mature writer, but it really should be left in the category of juvenilia. You will never bring your own original creations to vivid life as long as you rely on that crutch.

One thing that should be noted: writing fanfic can be used as an exercise to help an author through a writer's block... or get some new ideas/good lines.

Fanfic, though, is rarely as good as canon work. If it is as good, then it's like what Kitoba said - by a different author.

Fanfic shouldn't be confused with parody, though.