Oracle ADF Architecture

In simple:

Applications built with the best practices using the Fusion web technology stack acheive a clean seperation of business logic, page navigation, and user interface by adhering to a Model-View-Controller architecture.

• The model layer represents the data values related to the current page, along with the model-level business rules, security and application logic used against the data values.
• The view layer contains the UI pages used to view or modify that data.
• The controller layer processes the user input and determines page navigation.
• The business service layer handles data access and encapsulates the business logic.

The other modules that make up for Fusion web application technology stack are :

1. ADF Business Components: which simplify building the business services. 
2. ADF Faces: which offers a rich library of ajax-enabled UI components for web applications built with JSFs.
3. ADF Controller: Which integrates JSF with ADF Model, ADF Controller extends the standard JSF controller by providing additional functionality, such as resuable task-flows that pass control not only between the JSF pages, but also between other activities, for instance method calls or other task flows. The ADF Controller also works with the data model to ensure the proper data is used for each request or region.

Source : Developers Guide