In my understanding, Oracle ADF (Application Development Framework) is a technology for visual and declarative Internet application development. This "conceptual framework" is implemented by JDeveloper, Oracle's main IDE for Java EE programming. ADF encompasses, abstracts and sometimes re-defines a huge variety of concepts from all the different technologies used by a multiple-tier architecture, going from the database to JSF, through EJBs and Ajax (itself not a proper technology, but a combination of different Web programming techniques).
An Oracle White Paper, published in July 2009, describes the ADF architecture and its benefits over other frameworks (Oracle Application Development Framework Overview).
Still I have several puzzles, for instance: What are ADF business components? Enterprise JavaBeans? Something else or something more?