What Is Cluster Analysis? 

Cluster analysis is an exploratory data analysis tool for solving classification problems.  Its object is to sort cases (people, things, events, etc) into groups, or clusters, so that the degree of association is strong between members of the same cluster and weak between members of different clusters.  Each cluster thus describes, in terms of the data collected, the class to which its members belong; and this description may be abstracted through use from the particular to the general class or type.

Cluster analysis is thus a tool of discovery.  It may reveal associations and structure in data which, though not previously evident, nevertheless are sensible and useful once found.  The results of cluster analysis may contribute to the definition of a formal classification scheme, such as a taxonomy for related animals, insects or plants; or suggest statistical models with which to describe populations; or indicate rules for assigning new cases to classes for identification and diagnostic purposes; or provide measures of definition, size and change in what previously were only broad concepts; or find exemplars to represent classes.

Whatever business you're in, the chances are that sooner or later you will run into a classification problem.  Cluster analysis might provide the methodology to help you solve it; and Clustan could provide the professional software you need for that task.