Since this is an OS based on a browser (similar to ChromeOS),the concept UI is taken fron traditional browser point of view, but extended to serve the function of full feature OS. You can see some traditional browser URL bar, a tab (usually put at the top on modern browser), but combined with notification box on the right bottom and usual icon drawer for app launcher.

In todays convergence devices, it is become more important to have scalable UI/UX for different form factors.

Since the beginning, the concept is build on to of this idea. The image above is an example that the UI can be scaled down to smartphone screen size. 

What you see here is a main UI element called "stream". It give user whats matter to hir/her. It can be their social network feed, weather, stock information and so on. Last week (11-5-2013) Nokia Asha 501 also presented with similar concept called "fastlane"/

Again, the apps also need to be scalable. The solution is a cobination of putting content in a row of in a collumn. Here you can see the implementation of "stream" in smartphone screen size.

This is the example of full screen browser UI. Note that on the left bottom of the screen there is a small icon represent the website your at. If pressed, it will give you access to other tab, or back to normal viewing mode

This is an expose like UI. Derived from Firefox desktop browser ability to zoom out all the open tab and grouped whats related.