I have written a tutorial document on OWL
<a http://www.volcano.net/~rhm/doc/ClassAndSubclass.txt>
"Class" and "Subclass"</a>

Here are some hints to help you decide whether
or not you should read it.

Contents
========
Abstract
Class: meaning vs. programming
Subclass: exclusive vs. inclusive
Example: OWL Classes
Remarks
References

Abstract
========
The purpose of this document is to clarify the meaning
of "Class" and "Subclass".  These concepts are
introduced with a simple example, and then applied
to OWL Classes.

Remarks
=======
The OWL Class is primarily a data structure
used to represent a concept; it is an essential
element in the programming hierarchy.

In the context of the meaning hierarchy,
the OWL Class [1] simply denotes an entity.
The KR version of OWL [2] omits statements
such as:

Property isa* Class
Thing isa* Class

because they are contradictions.

A more comprehensive meaning hierarchy,
tabula rasa, is given in [3],[4].
The KR language and its implementation
are described in [5],[6].
