In library school, we learn about LC subject headings and other structured, formalized ways of categorizing information. This is important, and no one is arguing that we should give up on subject headings altogether, but I think librarians need to at least understand the way users are organizing their own information. Tagging, exemplified by social bookmarking sites such as del.icio.us, allows users to create their own labels and then compare them to the labels that others have used.
When I browse through the bookmarks in my network, I use the tags to help decide whether it’s something in which I’m interested. Since site titles can be misleading or uninformative, and most people do not regularly add user notes for their bookmarks, tags are often the only way I have of judging the usefulness of a site without clicking through.