Tagging is an open and informal method of categorizing that allows users to associate keywords with online content (web pages, pictures & posts). Tagging is completely unstructured and freeform, allowing users to create connections between data anyway they want.
Tags are like subject headings; however, there is no thesaurus or authority control. Users assign the tags that are meaningful to them. This sounds like chaos to most librarians; however, it actually works well in the web environment.
You can use tags in Flickr, Delicious, Library Thing, and a number of Web 2.0 applications. Some OPACs have started supporting user tagging.
You can read more about tagging in Wikipedia: http://en.wikipedia.org/wiki/Tag_(metadata)
You don’t have to sign up or explore any application for Thing 13. Instead, write a blog post on tagging. Does tagging belong in the library? Does it replace or complement subject headings? Should we encourage or discourage the practice?
Enquiring minds want to know...