This is YouTube Gaming. It's a mobile app that serves as your phone or tablet's portal to Google's war on Twitch's video game dominance. It is excellent.
Even for something still in testing (neither the service or app has seen a public release), I'm very impressed with how clean, simple and functional the app is, to the point where I think this is way better than Google's actual YouTube app.
Below is a little guided tour (you might want to play it full-screen so you can actually read it).
As I mentioned in the video, you can really only mess around with this on Android at the moment, via a leaked APK; you can get that here if you want to try it out.