Multimedia is a single story told by various mediums compared to transmedia which is multiple stories set in a single universe. Each is told by different media and they complement each other to form an overarching narrative. By reading this description, it's still rather hard to identify between the two because they are both quite similiar in definition. They both also are a form of the distribution of media. The key element is how this distribution is taking place.
Now Multimedia, like stated is a single story. This single story, to help you remember, is kind of like a single mind. It is a story that can be told to one person in many different ways. You take a show such as Pokemon and turn it into a toy, a video game, and a card game. The idea is still pokemon but it has been evolved into different sources of media.
Transmedia has much similar idea in mind because it still pertains to stories, but instead of a single story, it involves multiple. There is an engagement that comes from transmedia, that we do not get from multimedia. As One3 says, "Transmedia creates multiple points of entry into the story."
Transmedia allows a connection of the story with the fans who are entertained by this story. It allows them to have this connection whether its from comicbooks, multiplayer games, and web discussion. It brings the story into the real world to be communicated with those people who have brought it into reality.