To me, metadata is a complex way to define objects. Metadata is a way to define an object, concept, or idea by multiple different facets. Tagging, on the other hand, is almost like a single field in a metadata schema. And it’s not a very official field at that. I like to think of tagging as a field which defines the social context of the object, almost as a gauge of the social mindset.
Tagging is hardly a good way to categorize objects, because of several problems. Multiple ways to describe (or even spell) one object lead to a huge discrepancy of descriptions. But tagging is useful in establishing how people think and feel about these objects. A social context is an effective way of situating objects, that lend to its definition. So, while I think most tagging is not a way to directly label and categorize objects, it can add some of the emotional nature of social tagging.