Tags are metadata without labels. Imagine online library catalogs, if instead of keeping separate indexes for authors, titles and subjects, we could only access these bits of information through one general index. We're calling this 'tags', but in a library catalog, we called it a keyword index, and in the days of yore we called it a card catalog.

In the context of reference, it is likely that we will use most tags to describe questions. Most tags will be subjects, descriptors, labels, etc. We can also use them to claim questions or refer them to one another, by tagging a question with the name of a library or librarian. And we can use them for whatever else we want.

This is an experiment, but I think tags will be useful for us.