Taxonomy is the science of classifying things. It is often modeled after the formal system of Linnaean taxonomy in biology, which is a strictly hierarchical system. But not all objects or subjects fall into neatly hierarchical patterns of categories, and in such cases non-hierarchical taxonomies may be appropriate.
It should be noted that even where the things being categorized are real, natural objects, the categories used are human inventions, and generally represent a theory about how the objects should be classified. Often more than one theory can be rationally supported. This is even more true when classifying human artifacts.