The tree shows the classes of the DBpedia Ontology.
Due to the size of the ontology tree, it is not possible to look at a complete overview. Even if zoom mechanism was introduced the overview would results confusing. Furthermore, the overlapping edges make difficult to read relationships between classes.
The Mike Bostock Cluster Dendogram has been used for constructing the visualization.
In his original block, Bostock gives this diagram the name "cluster dendrogram". Yet, there are no clusters in his example, nor the word "dendrogram" is used to indicate the result of a hierarchical clustering (this is the definition of dendrogram I know).