A defining characteristics of Web 2.0 is tagging whereby users add their own metadata to content they produce, consume and share. On Flickr (www.flickr.com) and Deli.icio.us (del.icio.us) for example, any user can attach tags to digital media items (files, bookmarks, images). The aggregation of tags create an organic, free-form, ‘bottom-up’ taxonomy. Folksonomies are flat (they have no hierarchy and show no parent-child relationships) and, critically, are completely uncontrolled. A key implication of their lack of structure is that they do not support functions such as drill-down searching and cross-referencing.
Chaffey, D. and Ellis-Chadwick, F., 2012. Digital marketing: strategy, implementation and practice (Vol. 5). Harlow: Pearson.