Fanfiction
A story written by a member of an existent intellectual property's fandom, usually written in that property's universe, and including characters, places and events from that universe. However, sometimes, this is not the case.
A brief overview and history[edit]
Depending on your definition of fanfiction, the earliest examples of it might go back to the earliest examples we have of literary works, going back thousands of years. Since most legends and tales were never recorded and only passed on orally (sitting around a campfire and someone telling said stories from memory), these stories tended to evolve over the ages by storytellers not getting details down right or adding details of their own. The oldest surviving examples of written fanfiction are by some considered to be later versions of the Arthurian Myth as they contain a lot of details that are either abscent in older versions of it and some stories being a lot younger than the Myth they are based on. Of course, this brings with it fierce dabate on what material is canon and what not that goes back centuries, so we best leave it at that.
Fanfiction as we understand it nowadays came about in the 1930s, when fans of Sherlock Holmes banded together around magazines and literary clubs in the UK and the US to write their own cases and continuations of Arthur Conan Doyles original stories, something the man himself would probably not have approved of, as he came to see Holmes popularity as a burden that overshadowed his other works (of which there are equally many) and had Holmes killed off in The Final Problem, only to bring him back because his publisher forced him to, sparking the age-old debate in fanfiction communities over authorial intent.
The opposite side of this coin came with Roddenberry and Star Trek, who sheparded his fans lovingly into writing their own stories for the setting in fan magazines, starting in the 1960s, creating the first platform where fanfiction authors could exchange their stories with other like-minded individuals. It was in these stories that the Mary Sue first rose to prominence, the name itself coming from a Star tTrek fanfiction story that was itself a satire of the kind of self-insert characters that would come to be known by the name and become a persistent blight upon the body of Fanfiction forever after. The big boom for fanfiction arrived, naturally, with the widespread adoption of the Internet in the mid-1990s, especially in the Harry Potter and, a bit later, Twilight Fandoms. Fanfictions reputation for producing mostly derivative garbage (and porn, lots and lots of porn) also comes from these fandoms, simply because these were mainly teenagers writing them, and well, teenagers being not particularly great or experienced at anything produced the results that were to be expected. This reputation also caused a lot of rifts between authors and their audience, since many of them were quite protective of their works and didn't want fans meddling with their own personal vision for their stories. Nowadays authors have mostly given up on actively fighting against fanfiction, at most reacting disapprovingly but still accepting that online fanfiction sites are here to stay.
Fanfiction communities in themselves have also changed quite dramatically, since it has transformed from being mostly childs play in an established world to its own genre of literature. A lot of modern fanfiction has stopped considering the canon version of the work it is based on even to be the baseline, with their authors mostly cherrypicking their own personal favorite characters for their stories and changing everything else. This development has also given rise to the predominant use of tropes in fanfiction in order to describe the content of any given story. That in itself has attracted critism from more academic circles, since books, especially YA novels, now tend to be marketed more around tropes and less the stories themselves than it has been in the past, making the stories increasingly formulaic and less able to transport deeper themes and meaning. The utter disregard larger fanfiction communities and the prominent voices within them hold towards classical literature and analysis of such has also attracted a lot of criticism, with former fanfiction authors like, for example, E.L. James (of 50 shades of grey fame) being utterly oblivious to the story and characters they wrote or the core themes they based their work on.
It bears mentioning that quite a lot of new authors have used writing fanfics as practice for their own later craft, or if they were particularly lucky, even got their fanfiction published as original works. It is, overall, just as literature itself, a bit of a mixed bag, since there is a lot of garbage out there, making the good stuff really stand out. If you want to practice your craft or skills as a writer, starting with Fanfiction is not at all that bad of an idea.
When fanfiction is good[edit]
Sometimes fanfiction can be good: An interesting story in an existing setting, An alternate universe the fan writer wishes to explore, Exposition or lore fashioned into an easier consumed narrative, and more. Because of this, it is possible for fanfiction to be surprisingly well written. The blessed individuals that grace /tg/ threads with this rare treasure are affectionately dubbed Writefags.
BAD fanfiction[edit]
BAD fanfictions are usually stories that are little more than the writer's sexual fantasies put to paper (E.G. a certain story involving gay orks or making your waifu fantasies with Elesh Norn come true, And that's barely the tip of the iceberg.) These are called FapFictions, and they are unnervingly prolific.
And then there are the legends. Fanfictions so bad, so mind-blowingly awful that they have taken on a life of their own and much like famous bad movies like Troll 2 and The Room have gained their own cult followings. My Immortal by Tara Gillespie is one of the most famous with its terrible spelling, OOC characters, batshit plot and much, much worse.