fan fiction
... is the term generally used when people draw on an existing fictional characters and settings (in book, tv or film format) to write a story of their own.
There's a huge universe of fan fiction out there, writers and readers both. There are some very talented people writing Farscape fan-fic (more about that in another post); I know there are many websites where people post and read fan-fic for Friends (my daughter reads them on-line and tells me about those stories).
It's an interesting phenomena. As an author I approach it with equal measures of curiosity and trepidation. On the one hand it's tremendously flattering that the characters conceived and created in the author's mind have taken on such life that they go out into the world on their own and have new adventures. On the other hand, it's a little bit like knowing that you have a child out there dating somebody you don't know anything about. Anybody with kids knows that this is nightmare material.
In technical terms, fan-fic is probably a violation of copyright, but I have never heard of an author prosecuting, and in fact I think that would be stupid. If a few people get together to tell stories about Author X's people and places, and there's no attempt to sell or make money from those efforts, then I don't see that there's any problem. If a fan-fic writer tries to claim a character, of course, then things would get a little complicated and uneasy. All of the fan-fic I have seen, however, makes a great effort to establish who owns copyright, and that the stories presented are not meant to be any kind of infringement.
In fact, some authors and writers encourage a certain kind of fan-fic; that's why they sell action figures -- so we can sit at home and construct our own scenes between characters, the way we'd like to see them happen. And sadly, while the John Crichton action figure will never be John Crichton, role-playing is not unhealthy and might even be therapeutic for the... truly engaged viewer. Note I haven't used the 'f' word here.
A related but distinct area is screenwriting. Ann posted this query:
... would you mind if I practiced learning screenwriting skills by using your books? I would not be looking to sell the screenplay - just an exercise for myself.
The reason this is more complicated than simple storywriting is that the screenplay rights for the Into the Wilderness are under option to a producer, and thus they are not exclusively mine to make decisions about. Certainly if the deal begins to come together, it's very unlikely that I would have anything to do with the writing of the screenplay (or the casting of the actors, either). So if somebody has an idea for writing a few scenes from one of the novels and is doing so for their own enjoyment and as an educational exercise, I see no problem with that -- but that's about all it could ever be.