• 1.1 By story continuity
  • 1.1.1 Storyline is not complete without subsequent works.

    Most commonly found in a trilogy. J.R.R. Tolkien's ur-trilogy is a bit of a misfit here, considering that he wrote it as a solitary work and it was later split in three. Guy Gavriel Kay's The Sarantine Mosaic is a fine example of duology, while you can take your pick of fantasy series that stretch one story over four or more books. As we speak, George R.R. Martin, Robert Jordan, and Steven Erikson are breaking bookshelves worldwide. I see science fiction as less notorious for this than fantasy, although it does happen. Consider Kim Stanley Robinson's Mars trilogy, or even his Three Californias, which were narratively complete on their own but tightly linked on a conceptual level.

  • 1.1.2 Storyline is complete. Subsequent works occur in the same universe, perhaps using the same characters, but there's a new crisis.

    Very common. This could include future histories, like Ursula K. Le Guin's Hainish books, or more direct continuations of one person's story like Richard Morgan's Takeshi Kovacs series. I would probably put the five books of the Hitchhiker's Guide to the Galaxy trilogy here, as well as most sequels in the movies (very few movies leave a story unfinished).

  • 1.1.3 Major elements of the storyline are complete; an overarching story continues.

    This is how TV shows like Babylon 5, Buffy, and Battlestar Galactica keep the audience both satisfied and wanting more. [Thanks to Matthew Rees for suggesting this category].

• 1.2 By chronology (this is arguably a subset of story continuity, but breaks out enough on its own to warrant its own category)
  • 1.2.1 Subsequent books pick up storyline immediately after the previous one

    This happens most often in trilogies, but it also happens to stand-alone books that follow one another (in other words, it could include both 1.1.1 and 1.1.2). Incidentally, the strongest version of this category would be those recent books that have been split in half due to publishing reasons.

  • 1.2.2 A long gap in chronology.

    A perfect example is the expanding gap in Arthur C. Clarke's books: 2001, 2010, 2061, and 3001. It's the same series but a thousand years have elapsed between the first and fourth book. Sometimes the gap can be shorter. For example, over the course of Jonathan Stroud's excellent Bartimaeus Trilogy, the main character grows from a boy to a young man.

    Two other examples would be the aforementioned Ender novels - Card covers many years because he sends Ender on a slower-than-light trip through space - and Stephen Baxter's Evolution, which has to cover millions of years to live up to its title.

  • 1.2.3 Prequels. Subsequent books take place before events of the original book.

    The mind immediately thinks of Star Wars. That's enough to make me blank out on any other examples.

    As Wikipedia points out, prequel is a portmanteau word from the 1970s. And they have a lot of examples over there.

  • 1.2.4 Simultaneous events.

    Much less common, likely because it's difficult to do well. One example is Orson Scott Card's return to the universe of Ender's Game with the sequel Ender's Shadow which retold the same events from the point of view of a different character.

• 1.3 By creator's relationship to the work
  • 1.3.1 Original creator.

    This is usually assumed to be the best case, although not always. Plenty of authors have beat their own series to death, while other writers have kept their artistic integrity in the process.

    As an aside, there are few writers who have not returned to a previous book in some way (at least writers with more than, say, half a dozen books to their credit). Robert Charles Wilson for one. Other examples?

  • 1.3.2 Family of original creator.

    I'm thinking here of the Dune prequels and sequels, written by Brian Herbert along with Kevin J. Anderson.

  • 1.3.3 No relation or for hire.
    • 1.3.3.1 Ghost-writing.

      It's not the original creator, but the reader doesn't know.

    • 1.3.3.2 Designated in some way by rights-holder.

      This would be vast majority of cases. I'm picturing the multitude of comic books on the shelves - Batman has been around for decades and the sequels are numberless, all authorized by those in control of the copyright.

    • 1.3.4 Sequel to works in the public domain.

      Stephen Baxter's sequel to The Time Machine. I'm also thinking of the more complex case of Alan Moore's League of Extraordinary Gentlemen, which was a continuation/reworking of half a dozen works in the public domain (it also falls under 3.2.3 below).

Technically this is a subset of category 3, but remakes by definition stay in the same medium so there's no conversion going on. Interestingly, the term "remake" is a pejorative one and applies to the movies, while a retelling is an author happily ripping off something like a fairy tale. What's with that?

• 2.1 By degree of similarity
  • 2.1.1 The exact same work

    Very rare - see Gus van Sant's Psycho for the only example. Most people wondered - what's the point?

  • 2.1.2 With noticeable variations

    This is the most common case. It could be a remake of a foreign film, or a film that is a few years old. It could be something like Margaret Atwood's recent The Penelopiad (itself a contribution to an interesting series retelling old myths) or one of the many Ellen Datlow fairy-tales-retold anthologies like the recent Swan Sister.

  • 2.1.3 Wildly different

    The current example that springs to mind is Battlestar Galactica. It's so different than the decades-old TV show, its ostensible template, that fans of the old show felt that it should be titled something else, rather than saddling their favorite franchise with this unrelated junk. I happen to disagree, but I see their point.

Conversions from one medium into another

• 3.1 By similarity to source
  • 3.1.1 Really close

    The movie versions of Harry Potter and the Sorcerer's Stone or Sin City are most often cited as adaptations that were extremely close to their source materials. To my mind, they were both too close, and didn't take enough advantage of the inherent strengths of the new medium.

  • 3.1.2 Reasonably close

    Most common. And this makes the most sense to people: a movie isn't the same thing as a book, and shifting things around or leaving out some details is a wise approach. I would say that Peter Jackson's Lord of the Rings, for all of the warts and offences to fandom, is a shining example of how to not screw things up.

  • 3.1.3 Wacko

    Technically, Battlestar Galactica could go into this category. A more famous example would be Blade Runner, which ditched a lot of the material from the book it was based on, Philip K. Dick's Do Androids Dream of Electric Sheep, and even inverted Dick's most plainly argued points from the book. But what a movie! I would tend to judge on an ad hoc basis, rather than categorically saying that an adaptation has to be reasonably close to its source.

• 3.2 By relationship to source

  . This category could go on ad infinitum, so I've kept to the main ones.

  • 3.2.1 Book/comic to movie

    This is probably the one that springs to mind, although considering the low number of genre movies made a year, it doesn't happen to many books. And it generally does not bode well for the work in question. For every Spider-Man, there's a From Hell or Constantine.

  • 3.2.2 Novelization of movie

    Every movie has one. They are a strange species, I must say. What is the point? Are they a marketing tool?

    • 3.2.2.1 Crummy novelization

      Just a depressing makework project.

    • 3.2.2.2 Unique novelization

      I can think of two cases, The Abyss and 2001, and these two make the category worth breaking out. In both cases, there was a strong director and a strong writer involved.

  • 3.2.3 Comics of a book/movie/TV show

    Buffy comes to mind, as there have been numerous comics about the Buffyverse. This would include things like the Illustrated Classics and other stuff for kids.

  • 3.2.4 Book of a radio broadcast

    Hitchhiker's Guide to the Galaxy makes this a category all on its own.

  • 3.2.5 Radio broadcast of a book

    Orson Welles' War of the Worlds is the biggest example here but other less famous have also been made.

  • 3.2.6 Computer game of a RPG

    Oh boy, is this ever an overdone category. Thankfully there have been a few original role-playing computer games.

  • 3.2.7 Etcetera

    As mentioned, this could be a long list. I would also point out that popular stories tend to move through all of the categories until it's sometimes hard to remember where they started.