Introduction to OOP
Chapter 10: Subclasses and Subtypes:
To say a class is a subclass of another simply asserts that is was built using inheritance.
By itself, the subclass relationship says nothing about the behavior of the child in relation to the parent.
The term subtype is used to describe a class that matches the behavior of another class.
It is easy to build subclasses that are not subtypes. It is possible (although not as easy) to build subtypes that are not subclasses.
, Slide 10