Views is a user interface framework built on a type called, confusingly, View. Responsible for providing the content of our Aura windows, most of this code is also used by Desktop Chrome on Windows. Only the NativeWidget implementation differs (for now). Views also provides a set of useful reusable controls like Buttons, Menus, etc.
Views is somewhat analogous to Aura, with a native host (NativeWidget implementations), a View hierarchy within a RootView that handles event dispatch.
The primary difference is that this system was designed and built before beng developed a healthy skepticism of is-a relationships, and so you accomplish rendering, event handling and layout here by subclassing View, rather than implementing a delegate interface as in Aura.