Hosted Web Apps
Boom, it's an app.This demo really impressed me. Not only does this mean you can create an app from an existing web application without many changes, but any changes you make on the web are instantly available within the mobile app without the need to redeploy. As Michael mentioned though, this is really only going to work well for web applications that utilize responsive design.
Lastly, and almost as cool, you will also be able to access Windows APIs from this app. This means the website you just turned into an app can also access device features like Notifications, Camera, Calendar - even Cortana!
NOTE: Building Universal Windows apps requires Windows 10 configured for development + Visual Studio 2015.