Tags and categories have different connotations. To me, tags are community, collaboration, flexibility, fuzziness, visibile metadata. Categories are authority, rigidity, structure, taxonomy. Tags can be found inside content (), categories are separate, controlling entities. Content owns tags. People own tags. Categories own content. Authority owns categories.

Beware of vague naming — some software mistakes one for the other (e.g. Mediawiki categories are in fact many-to-many).

The organisational technique used doesn’t only have technical and usability implications, but social and philosophical (or pseudo-philosophical?) ones.