Taxonomies originated in the domain of biology. Biologists use taxonomies to create a hierarchy of species. A taxonomy always takes the shape of a tree, where one taxon may have multiple branches but only one root (or parent) taxon. This makes it possible to classify an object (animal, document) unambiguously.
A taxonomy and a thesaurus are very similar. However, a thesaurus allows non-hierarchical relationships (the Relates to relationships). From the perspective of content classification, we may consider a taxonomy to be a narrower version of a thesaurus.