Henry Jenkins of MIT is the first to coin and describe "Transmedia" and the "Transmedia Narrative." In his "Transmedia Storytelling 101" (http://www.henryjenkins.org/2007/03/transmedia_storytelling_101.html), his first of ten points is, "Transmedia storytelling represents a process where integral elements of a fiction get dispersed systematically across multiple delivery channels for the purpose of creating a unified and coordinated entertainment experience. Ideally, each medium makes it own unique contribution to the unfolding of the story. So, for example, in The Matrix franchise, key bits of information are conveyed through three live action films, a series of animated shorts, two collections of comic book stories, and several video games. There is no one single source or ur-text where one can turn to gain all of the information needed to comprehend the Matrix universe."
Apart from being a separate point of entry, each medium has it's own best practices that if thoroughly understood, can dimensionalize the central theme, or idea, making the whole story far more rich, interesting and powerful.
The slide show presented here, by Big Spaceship, is one of the better Transmedia presentations we've seen.