In creating the description of the problem domain, the first step in producing an object-oriented design, you are mapping from the real world to the virtual one. In traditional programming paradigms such as structured methods, the emphasis is less on trying to mimic the real world and more on creating a program.
In order to maintain the OO paradigm, you need to retain a one-to-one mapping between the real-world objects and the objects in the system. So, if you are trying to create an OO design for a problem domain that contains a user interface for an address book, you have a clear list of possible objects based on observing a real-world address list:
Edit areas to view and edit details
Manipulation buttons (Previous, Next, ...