Fanfiction is where someone who loves to write, usually someone who has not been published before, writes a story based on a previously established work of fiction using the characters or the world. It could be a retelling of the same story or a different focus or perspective. It could also be entirely new characters in an established world or that kind of thing. Usually widely popular fictions are chosen for fanfiction.

For example, the author mentioned E.L.James who took the Twilight series, and keeping the dynamics (not really) and tropes, turned it into an erotica story. Other fanfiction writers might retell Twilight with Bella ending up with Jacob, or with Bella as the vampire and Edward the human, or take another pair like Jasper and Alice and make them the focus, or set the story in early 20th century New York. The possibilities are endless.

Star Wars and Star Trek have also been big fanfiction favorites. Harry Potter, Supernatural, Naruto, etc. Basically any fictional work that gains enough popularity to have a fandom will have fanfiction created from it sooner or later.