While all objects of a class have the same attributes, each object has its own copy of the attribute value.
All Person objects have the name attribute but the value of that attribute varies between Person objects.
However, some attributes are not suitable to be maintained by individual objects. Instead, they should be maintained centrally, shared by all objects of the class. They are like ‘global variables’ but attached to a specific class. Such variables whose value is shared by all instances of a class are called class-level attributes.
The attribute totalPersons should be maintained centrally and shared by all Person objects rather than copied at each Person object.
Similarly, when a normal method is being called, a message is being sent to the receiving object and the result may depend on the receiving object.
Sending the getName() message to the Adam object results in the response "Adam" while sending the same message to the Beth object results in the response "Beth".
However, there can be methods related to a specific class but not suitable for sending messages to a specific object of that class. Such methods that are called using the class instead of a specific instance are called class-level methods.
The method getTotalPersons() is not suitable to send to a specific Person object because a specific object of the Person class should not have to know about the total number of Person objects.
Class-level attributes and methods are collectively called class-level members (also called static members sometimes because some programming languages use the keyword static to identify class-level members). They are to be accessed using the class name rather than an instance of the class.