Otherwise known as
fanfic, fan fiction may be written singly or working in groups. One of the larger appeals of fanfiction is the ability to
"play in a larger sandbox", working with characters and situations created by others, amplifying and changing them for one's own purposes. Sometimes, as in
Robotech: The Misfold, a fanfiction may itself be a shared universe, with an
editor and
writers working together.
Fanfic has a long and distinguished history, beginning decades ago with the emergence of Star Trek as a cultural phenomenon and new mythology for our time. At that time, fanfic writers put together fanzines that were xeroxed and passed by hand to a few dozen or a few hundred pairs of eyes. With the rise of the Internet and the World Wide Web, however, fanfic could be distributed to tens or hundreds of thousands of willing readers everywhere. This has caused some problems relating to copyright and trademark.
One could argue that some of the ethics of fanfic are actually much older. For example, Shakespeare based his plays on history and other events, back before the legal fiction of copyright changed things. Maurice Leblanc's Arsene Lupin stories often featured cameos by a thinly-disguised Sherlock Holmes.
One of the worst things you can ever ask a fanfiction writer is "Why don't you write something original?"