Glossary of Terms for Web 2.0...Tags

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Tags
At some level if you have a body of knowledge that supports your industry or profession, you will need to utilize "tags" to label your content. Tags are the common denominator that is part of the plumbing of your online asset gallery.

From Wikipedia:

Usage
A tag is a keyword which acts like a subject or category. A keyword is used to organize webpages and objects on the Internet. Each user "tags" a webpage or image using his/her own unique tag. An image or webpage may have multiple tags that identify it. Webpages and images with identical tags are then linked together and users may use the tag to search for similar webpages and images

Tags can be used to specify properties of an object that are not obvious to the object itself. Tags can be used to find objects with similar properties or to organize objects.

Tags are often used in social software and Web 2.0 pages. The method of allowing open categorization of the Internet is often referred to as Folksonomy.

Bookmark Tags are used in Flock (web browser) and will surface in Mozilla Firefox 2.0x

Semantics and association
Tags do not necessarily define their semantics, but are often interpreted as being related to the concepts which are popularly associated with their contents. For example, the tag "apple" might refer to the fruit, Apple Computers, the Beatles' music label, or Gwyneth Paltrow's baby. It is the user, not the computer, who parses out which items are relevant to their needs.

Due to the fact that different people will use different tags to describe the same item, the associational nature of tagging makes it an inherently ambiguous system. For example, some users may tag a webpage about Apple's operating system "apple", whilst others may tag it "OSX" or "Mac" or "Tiger". This aspect of tagging has received criticism in relation to traditional taxonomies because no rules exist to standardize the tags people use, which creates a "messy" and scattershot system of categorization. For some, however, this freeform messiness is precisely what makes tagging dynamic and democratic.

Comparison with other categorization schemes
Tagging requires a group of people to cooperate in order to organize information. This idea is known as Taxonomy. Unlike hierarchical methods of categorization, tagging lacks concrete guidelines to define how an object must be classified. One tag may link to seemingly unrelated websites. Since tags are not issued by one authoritative entity, classification by tags is personalized. What may fall under a specific tag to one user may not relate to another object with the same tag by another user. This results in an evolving categorization scheme. Categories are established through time by many people.

Other forms of categorization:

Hierarchy - not always applicable, but often more accurate
Ordered list - seldom applicable for large object sets
Network - Always applicable, but may result in enormous taxonomies to be able to define all types of relations

Example websites that use Tags
Del.icio.us - A social bookmarking site that allows users to bookmark many sites and then tag them with many descriptive words allowing other people to search by those terms to find pages that other people found useful.
Gmail - A webmail site that was one of the first to allow categorization of objects using tags, known as "labels" on emails.
Flickr - A service that allows users to tag images with many specific nouns, verbs, and adjectives that describe the picture. This is then searchable.
Technorati - A weblog search engine.

Source: http://en.wikipedia.org/wiki/Tags