These are just terms defined for user-interface of your application and the usability of your application. The User Interface would define the actual interface of your application such as buttons the design of your application and how it is rendered on different screens and platforms. Whereas the Usability of your website defines how much the application is easier for the targeted users of that application. You can think of them like the user-interface defines the design of your application whereas the usability defines how much easier it is for the user to use that design to perform a particular task in your application's context. For more on this topic and according to the .NET platform you can read this document for more information. https://msdn.microsoft.com/en-us/library/ms997578.aspx