Since we are still in the Christmas spirit, I'm going to use a very Christmassy example to explain Transmedia storytelling.
The Elf on the Shelf.
It started out in 2005 with a book about a little elf. Who comes to the home of children to help Santa with checking who is naughty or nice. He musn't be touched, because he carries Christmas magic, that is very fragile. And during the night, he returns to the North Pole to report to Santa. Returning in the morning for a new day among the family.
Shortly after the book there was an animated movie about the Elf.
And now, you can adopt your own Elf. Who lives in your home. And returns in the night to the North Pole. So you never know where he will show up the next morning.
Some Elfs tend to make a mess out of their home. Others just move around the home to see what is going on.
Through internet you can write to Santa, where he might also reply.
And there is a very lively community on Pinterest, where all the mums are sharing the trouble they have with the elf. Because he himself can be very naughty.
So in terms of storytelling the Elf has a book, an animated movie, a website, stores, the elf himself and a big community of fans (and haters) who share their stories. Those are a lot of various platform to tell a story.
And with the digital media, it is really easy to share the story. I came across it on Pinterest, never heard about it before or even having seen the elf himself. But the story did come to me and now I'm sharing it with you.