Definition: An UI approach consists of the basic goals, ideas and principles of the UI.
It is often tried to be formalized by UI guidelines as published by many desktop environment vendors, but these documents only cover a very small subset of the UI approach.
On a high level, you can organize the UI after
Use-Cases: Action-oriented UI. To be seen in many COBOL-apps. E.g. "Print Transaction report" -> "Enter Account Nr.".
Objects: To be seen in many GUI-apps. E.g. "Person" -> "Account" -> "Transactions" -> "Print".
On lower levels (exemplary for 2D GUI):
Use of Drag&Drop
Which MDI strategy (childs, top-level windows, tabs etc.)
Use of real-world metaphors (e.g. trashcan)
Knowledge about UI guidelines is now not anymore stored in the programmers mind and hardcoded in the application, but moved to the OOUI Kit on the server. The kit performs the not-so-easy task of creating a good UI. The goal is to create a UI nearly as good as a hand-crafted one.