Myth #1 in Henry Jenkins's 7 Myths About Transmedia is that "transmedia storytelling refers to any strategy involving more than one media platform." It doesn't, and the following attempt at a classification of *media storytelling approaches could be useful for understanding the difference:
- A single story is told concurrently via different media, with the core narrative being supported by artifacts spread out across many types of media. None of these artifacts (except maybe one core piece?) can tell a (the?) story on its own, and the narrative can't be consumed in the absence of the elements. Many ARGs labeled as transmedia actually seem to be multimedia. Many consider traditional merchandising (think Happy Meal toys) to be a form of transmedia storytelling, but I doubt it can be classified even as multimedia.
- A single story is interpreted independently in different media. Consider The Lord of The Rings or Harry Potter books and their movie incarnations. Consuming the story in one medium can enhance one's understanding of the story told via the other, but each individual interpretation is self-sufficient.
- Multiple stories are set in a single universe, each is told via different media and they complement each other to form an overarching narrative. Example: The Matrix (one of Jenkins's original examples of transmedia storytelling), with the movie trilogy, the comic books and the virtual world all being self-sufficient but at the same time enhancing each other.
These approaches are not mutually exclusive, of course, and nothing prevents a media franchise from employing all three. Lost, with its combination of the TV series, the ARG, the video game, the board game, the novels and many other media artifacts is a great example of trans- and multimedia storytelling (examined in detail in Ivan Askwith's thesis).
To the extent that advertising is storytelling, this classification probably applies to our output as well.
This thinking is under heavy construction and I would love it if you could poke holes in it in the comments or elsewhere.