Collaborative tagging describes the process by which many
users add metadata in the form of keywords to shared
content. Recently, collaborative tagging has grown in
popularity on the web, on sites that allow users to tag
bookmarks, photographs and other content. In this paper
we analyze the structure of collaborative tagging systems as
well as their dynamical aspects. Specifically, we discovered
regularities in user activity, tag frequencies, kinds of tags
used, bursts of popularity in bookmarking and a remarkable
stability in the relative proportions of tags within a given
url. We also present a dynamical model of collaborative
tagging that predicts these stable patterns and relates them
to imitation and shared knowledge.