The document discusses different approaches to tagging and cataloging used by various systems including traditional library catalogs, Wikipedia, Flickr, del.icio.us, CiteULike, and LibraryThing. It notes that traditional library catalogs require determining a single subject heading, whereas user tagging allows multiple tags and is more flexible but less controlled. Systems like Wikipedia, Flickr, del.icio.us and LibraryThing allow social tagging by users for navigation and discovery while CiteULike is used for tagging citations.