I stumbled across a bit of text that clarified an earlier discussion on tagging:
Hierarchical: indicates a parent-child (vertical) relationship like cat and dog are children of mammals)
Association: indicates a "similar to" (horizontal) relationship like mammals is similar to animals.
Bingo! This is what people think of when they create categories and tags. Categories are hierarchical, and tags are associative. The problem is - they're both right and wrong.