Office Mix comes to us in the form of a plugin that slips into PowerPoint 2013, and with it, highly interactive presentations can be created. These presentations are not meant to be shown during a boardroom meeting, per se, but rather be handed off to people to enjoy by their lonesome. These presentations could be used for educational purposes, or for fun.
Like video, a Mix presentation will have a play button, but like old educational CD-ROMs, the presentations can be stopped at any point to partake in provided activities, such as interactive charts, quizzes, and so forth. After looking through the Knowledge Base, it seems uncertain that a user could pick and choose where in the presentation to go, but I'd have to assume that'd be a possibility.
A great feature of Mix is that PowerPoint isn't even required to be installed - at least not from what I can tell. Microsoft says that all of the latest major browsers are supported, as are tablets with Windows, Android, or iOS. Phones are also supported as far as being able to watch the presentation, but they'll be unable to participate in its interactive bits.
Mix is going to be free to those who have the latest version of Office, and if there's a downside, it's that a Mix can't be self-hosted; it has to remain on Microsoft's Office Mix website.
Unfortunately, it seems Microsoft didn't think to provide a demo of Office Mix in action, so hopefully someone, somewhere, will create one and share it to show us all what it's capable of.