What abstract data structure does it (a tag/category hierarchy) look like?
It’s sort of like a tree, but since it can have loops, I think it’s even more closely represented by a vertex-labeled directed (hierarchy) multigraph/pseudograph. (See http://en.wikipedia.org/wiki/Glossary_of_graph_theory) Just like how RDF data in general “represents a labeled, directed pseudo-graph”[1]. Or, to put it another way, a collection of tags/categories and their assignments can be represented by a bunch of triples…
