Transmedia is a telling the same story in different mediums. For example, the movie The Matrix is both a movie and a video game. Both mediums provide publicity for the movie.
Movies such as Star Wars, Harry Potter, and Lord of the Rings have been adapted into video games but hardly ever is the reverse popular. For example, the movie Prince of Persia was inspired by a video game and made into a movie. People that played the video game before viewing the movie came to the conclusion that the video did not meet their expectation.
Generally people like when a movie is adapted into a video game because they get to experience the story as a game. But when games are made into movies, they lose out of interacting with the movie.
Transmedia allows for participation on multiple mediums and if done right can positively promote media.