1. Apparatus for providing a visualized hierarchical display of categorized event data, said data being a collection of records, wherein each record is associated with an occurrence of a corresponding event and comprises a plurality of attribute/value pairs characterizing the event or an individual user associated with the event, the apparatus comprising: a processor; a memory connected to the processor and storing computer executable instructions therein; circuitry, connected to the processor, for accessing a plurality of data records, residing on a data storage medium, that collectively forms a dataset representing a collection of events and for applying the data records from the medium to the processor; and a display operative in conjunction with the processor; wherein the processor, in response to execution of the stored instructions: classifies the data records, based on the attribute/value pairs associated with each such record, into a plurality of mutually exclusive first clusters; determines a measure of similarity between each pair of said first clusters so as to yield a plurality of similarity measures for the first clusters representing the dataset; and forms, based on the similarity measures, a multi-level hierarchical cluster organization such that said first clusters are situated, as leaf nodes, at a lowest level of a hierarchy with second clusters being situated, as cluster group nodes, at successively higher levels of the hierarchy and formed as a result of selectively and iteratively combining clusters that are sufficiently similar to each other so as to form combined clusters in order to define a nodal set wherein each of the combined clusters replaces the clusters so combined to form said each combined cluster; and visually renders the hierarchical organization on the display.