From the Windows Phone Marketplace:
Finding Content: Just like on your Xbox, the Xbox Companion gives you instant access to the Xbox LIVE Marketplace catalog of games, movies, music and TV shows. I used it to search “Featured Content” and pulled up the X-Men: First Class.
Learn more about what you’re watching, listening to, or playing: Once I found the movie and touched the movie tile, the Xbox companion app took me to an info page that has all the related details about. And the content is really rich, too – for example, when I swiped to the “cast” page and touched an actor’s name, like star James McAvoy, I was taken to other movies starring him. From there, I could easily learn more about the other movies, their cast, etc. It’s an infinite road of interactive content.
Control & Play: Once I selected the content and hit play, the Xbox Companion flips into “controller mode” in which I could navigate my Xbox, or control video playback of your video. The best part here is that controls are virtually instantaneous – easily as fast as using my controller or a TV remote
Xbox Companion is free to all users and only available on Windows Phone.