In cluster analysis, one does not start with any apriori notion of group characteristics. The definition of clusters emerges entirely from the cluster analysis – i.e. from the process of identifying “clumps” of objects.  Clustering is used in many fields, including customer segmentation.  An airline analyzing its customer data, for example, might find that there is a distinct cluster of passengers with the following characteristics:  travel weekly, travel mainly one or two short-haul routes, book at the last minute, don’t check bags.