Categories as taxonomy, tags as folksonomy

See Rethinking Categorization

The theory is that categories are a taxonomy and tags a folksonomy. Taxonomies can be hierarchical, but folksonomies are, by definition, flat. However, in practice, tags almost always have an implicit hierarchy, which is also what research on folksonomies in the wild tends to find.

Well expressed, and the first I recall hearing of ‘folksonomy’, a useful distinction.