Fan fiction is a term that covers stories and novels that make use of the characters and settings from other people's professional creative work. You don’t make money from fan fiction, you write for fun, restoring missing scenes, patching plot holes, inventing prequels or sequels or letting the story play out quite differently than the original (canon) into an alternate universe (AU).
In its modern form, fanfiction might look like a new phenomenon; starting in the 60’s around TV series like The Man from U.N.C.L.E. and Star Trek, but in fact, the genre is as old as man himself. People have always built their own tales on the stories of others, that's the very essence of storytelling. Also, many classic masterpieces in literature are technically fanfiction. Virgil’s the Aeneid is based on a minor character in Homer's the Odyssey while Shakespeare plucked Hamlet and King Lear from historical and literary sources. Even the Bible can be said to contain fanfiction as the gospels of St. Matthew and St. Luke fill out plotholes in St. Mark’s, while St. John writes all three into a larger context.
While the fanfics of the 60's were published in s.k. "fanzines," internet is today the place where fans not only post but also meet and discuss their fandom and each others stories. Currently, fanfiction.net, the largest archive on the Web, hosts over 2 million pieces of fan fiction, ranging in length from short-short stories to full-length novels – and it’s only one site on many.
Naturally, the literary qualities of the stories vary a lot, going from utterly poor to beating the original itself. What they have in common is that they’re all creative expressions from people who can’t refrain from voicing themselves. On his blog10/7 2007, Mitchell Steele states: “These new authors are bringing in aspects that they, as an audience, wanted to see. The audience is taking over. This is their way of either continuing a story, or recreating an old one. The hero and villain can switch roles, or they could face new opposition. There are no rules. They are making them,” and in his article about fanfiction in Times 7/7 2011, Lev Grossman writes: “the fan-fiction scene is hyperdiverse. You'll find every race, nationality, ethnicity, language, religion, age and sexual orientation represented there, both as writers and as characters. For people who don't recognize themselves in the media they watch, it's a way of taking those media into their own hands and correcting the picture.”
As the term indicates, fan fiction is mostly written by fans; people who love the original story. But that doesn’t mean it’s uncritical; inspiration can as often come from frustration when canon fails to live up to expectations or leaves out parts the fan finds important. Fanfiction has no boundaries but carries from meticulously crafted stories to grose parody, it basks in everything from slapstik to sappy romance and goes from profound angst to unabashed erotica. "It's human nature to press at the boundaries of stories, to scrabble at the edges, to want to know what's going on just out of range of the camera," Lev Grossman writes. "What happens if I press this big, shiny, red button that says "Do not press"?