In this paper we describe a taxonomy of object-oriented
classes that catalogs each class in an application according
to the characteristics of that class, including the properties
of the data attributes and routines as well as the relationships
with other classes.  Our taxonomy is motivated by the fact that
the current research literature contains no formal methodology
for capturing the characteristics of a class.  To illustrate the
advantages of the taxonomy, we apply it to the problem of choosing
implementation-based testing techniques and, more importantly,
we show that our taxonomy can expose characteristics of a class
that remain uncovered by the chosen testing technique.