text: Figure 3.3: Representation of Classes containing Individuals 
  
  3.1.3 Classes 
  OWL classes are sets that contain individuals. They are described using formal (mathematical) 
  descriptions that rigorously define the requirements for membership of the class. For example, the class 
  Cat would contain all the individuals that are cats in our domain of interest.2 Classes may be organized 
  into a superclass-subclass hierarchy, which is also known as a taxonomy. However, taxonomies are often 
  trees. I.e., each node has only one parent node. Class hierarchies in OWL are not restricted to be trees and 
  multiple inheritance can be a powerful tool to represent data in an intuitive manner. 
  
  Subclasses specialize (aka*are subsumed by*) their superclasses. For example, consider the classes Animal 
  and Dog – Dog might be a subclass of Animal (so Animal is the superclass of Dog). This says that*All*
  *dogs are animals*,*All members of the class*Dog*are members of the class*Animal. OWL and Protégé 
  
  2 Individuals can belong to more than one class and classes can have more than one superclass. Unlike OOP where 
  multiple inheritance is typically unavailable or discouraged it is common in OWL. 