Tags are labels for something. There exist different classification systems, e.g. taxonomies like the ones used by librarians or folksonomies.
In the context of Web 2.0, Tagging means sticking keywords to something (a resource link, a web page, a picture, ...). The agents who can create tags are professionals, authors and users. One object can be tagged with as many words as one desires because there are no restrictions.
See also: controlled vocabulary (its opposite), metadata and indexing
Other usages of tag: A label in syntax used in markup languages like XML to delimit an element.
Thomas Vander Wal invented the expression folksonomies in 2004. They are "naturally grown" collections of tags, i.e. implicit taxonomies that are built by people creating and using tags.
“In contrast to professionally developed taxonomies with controlled vocabularies, folksonomies are unsystematic and, from an information scientist's point of view, unsophisticated; however, for Internet users, they dramatically lower content categorization costs because there is no complicated, hierarchically organized nomenclature to learn. One simply creates and applies tags on the fly.” (Wikipedia:Folksonomy, 18:45, 14 September 2006 (MEST))
Since Folksonomies are open-ended by definition, they do have some advantages.
Of course there are also disadvantages to folksonomies.
There are solutions to avoid some "noise", and supporters of tagging, such as Lars Pind (2005) and Marieke Guy and Emma Tonkin (2006), concentrate on this problem and propose some ideas to make folksonomies work better.
Apart from these practical devices that could be introduced to simplify the system, Marieke Guy and Emma Tonkin(2006) recognize the necessity of a tag etiquette. Though tagging is something retively new, a set of rules to organize this new world could be the simpliest solution to avoid lots of problems (see Semantic web). The community needs some fixed advice to follow in order to use this tool correctly and better exploit this possibility.
“Collaborative tagging describes the process by which many users add metadata in the form of keywords to shared content.” (Golder & Huberman, 2006).
(Collaborative) Tagging is used in many social software applications. Most of these are folksonomies, for example, to:
Rashmi Sinha (2006) in her social analysis on tagging recognizes that “The basic social formations supported by tagging are more like crowds than true groups.” The system connects people who share one word, but they do not really know each other, nor do they know what the ideas the others link with the concept they use for tagging something are. However, collaborative tagging transforms people's individual experience on the Web into a social one which makes the wide world of Internet a little bit smaller and more friendly.
Firstly, it's easy for users and requires only two steps of cognitive processing (Sinha, 2005). In contrast, filling in metadata forms is time-consuming, boring and difficult.
Secondly, metadata are ridid and don't work in the real world. An object is not always either type 1 or type 2, but can be both or in between.
Metrics and visualization techniques can put some "order" into a big "tag soup", e.g. show which tags are close (e.g. see tag clouds) and therefore create "natural taxonomies".
“Tagging repudiates one of the deepest projects our culture has undertaken over and over again: The rendering of all knowledge into a single, universal framework. The semi-chaotic state of the “tagosphere” represents the nature of our shared world better than the cool marble columns of the old mono-order ever could” (Weinberger, 2005, retrieved April 20, 2007).
Metadata taxonomies vs. folksonomies
Also see the way MediaWikis use categories, which is a sort of bookmarking. Each author can assign a category to a page. If it doesn't exist, the wiki engine creates a "wanted Category" page that needs to be edited in order to display the automatically generated list of pages. Categories can also be inserted into Categories (which turns it into a subcategory of the give category). Note: Categories in this wiki were made by a single user DSchneider. It's something in between a little folksonomy and reflection about our field.
http://insight.eun.org/ww/en/pub/insight/misc/specialreports/folksonomies.htm