Categories and tags are the two default WordPress taxonomies used to organise content, with categories serving as broad, mandatory groupings (like topics in a library) and tags providing specific, optional keywords for granular details. Categories are hierarchical (parent/child), while tags are not.

In practice individual sites will have their own guidelines for how to use them.

Personally, I always assign a post to just one category, and add tags as required. But the perennial problem remains - is a map of the Roman empire in category 'Map' tagged with 'Roman Empire', or in category 'Roman Empire' tagged with 'Map?.

This video makes some interesting points: