Tags are usually non-hierarchical fine grained descriptors of a resource. They are the opposite of categories which are usually part of a semantic hierarchy. Categories are really old-school, killed by the effectiveness of Search and the expanding mash-up universe.
However, while tags provide rapid access to resources and easier sharing of them, tags do not enrich knowledge. We can see this if we consider knowledge as one of the tiers toward wisdom: data, information, knowledge, wisdom. Tags are data on information, metadata. Categories are information on knowledge.
Since data is used to create information, the data on information, tags, can be harnessed to increase the information on knowledge by the automated creation of categories. We simply create the directed cyclic graphs of tags around resources and detect clusters. The naming of categories come from the resources themselves.
This is how the internet will wake up. It will create an ontology and it will act. Hopefully, humans are part of a necessary category.
Some possible related reading