The thought process embodied in the previous paragraph is called modeling . Modeling is the process of deciding what will go into your objects. In essence, you create a model of the world out of objects. The terms object and class are pretty interchangeable. Except that a class might be considered an object described in computer language, whereas an object is just an object.
Objects are somewhat situationally dependent.
The description of an object, and the class, depends on what needs to be done. If you were attempting to design a school course scheduling program, your objects would be very different than if you were designing a statistics program. Now back to the inventory system. You were reading about pens and how they had colors and other identifying features. In object talk, these features are called properties .