A node class is a class that,
-> relies on the base class for services and implementation,
-> provides a wider interface to te users than its base class,
-> relies primarily on virtual functions in its public interface
-> depends on all its direct and indirect base class
-> can be understood only in the context of the base class
-> can be used as base for further derivation
-> can be used to create objects.
A node class is a class that has added new services or functionality beyond the services inherited from its base class