Although Current Converter is a simple application, creating it illustrates many of the concepts and techniques of Cocoa programming. By now you have a much better grasp of the skills you’ll need to develop Cocoa applications. Let’s review what you’ve learned:

• Designing an application using the Model-View-Controller paradigm
• Composing a graphical user interface (GUI) with Interface Builder
• Testing the interface
• Specifying a class’s outlets and actions
• Connecting the class instance to the interface via its outlets and actions
• Implementing classes
• Building and debugging Cocoa applications

extract from:
Developing Cocoa Objective-C Applications: A Tutorial