Object-oriented programs are made up of classes. Classes correspond to real-world entities such as bank accounts or customers. An object is one instance of a class, much as 3 is an instance of a number and hello is an instance of an English word.
There are rules about how to add and subtract numbers, and they apply to all numbers. There are rules about how to write and say words, and they apply to all words.

Similarly, when you define a class, you are setting the rules for all the objects of that class. In this module, you will define the classes you named in the previous module. You'll learn about:

1. Responsibilities: what a class gets done
2. Collaborators: classes that work together to accomplish a task
3. Encapsulation: how to keep changes in one class from affecting others
4. The uses and benefits of traditional, structured programming
5. The uses and benefits of object-oriented programming (OOP)