An object is any entity, either real or abstract, that exhibits some well-defined behavior and that has a unique identity.
A class groups all objects that share the same set of properties and methods. Every object is an instance of one, and only one, class. A class describes the common characteristics of a set of objects. An object is often referred to as an instance of the class that describes it.
For details about defining your own classes, see "Defining Your Own Classes", in Chapter 3.