Tag (metadata)
Tags are pieces of information separate from, but related to, an object. In the practice of collaborative categorization using freely chosen keywords, tags are descriptors that individuals assign to objects.
Usage
Tags can be used to specify properties of an object that are not obvious from the object itself. They can then be used to find objects with some desired set of properties, or to organize objects. These features are exploited extensively in social software and folksonomies.
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. E.g., an audio tag of someone pronouncing the French expression for "fresh fish," "poisson frais," could lead a person to expect that the object is related to any of the following:
- Fishing
- Cooking
- A French person
- High school freshmen
- The movie "The Shawshank Redemption"
- Boisson fraîche (cold drink)
Syntax
Some tagging systems provide a single text box to enter textual tags. To be able to tokenize the string, a separator must be used. A popular separator is the space character. To enable the use of separators in the tags, a system may allow for higher-level separators (such as quotation marks) or escape characters. Systems can avoid the use of separators by allowing only one tag to be added to each input widget at a time.
Another syntax for use within HTML is to use the attribute rel="tag" to indicate that the linked-to page acts as a tag for the current context. eg to tag this page with 'folksonomy' you would add a link <a href="http://en.wikipedia.org/wiki/folksonomy" rel="tag">folksonomy</a>
More detail in the rel tag specification.
Comparison with other categorization schemes
- 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
Online services and their tagged objects
- 43_Things - shared goals
- Adzooks - classified advertising
- Del.icio.us - social bookmarks
- de.lirio.us - social bookmarks & notes (open source code & data)
- EVDB - events, venues, calendars
- Flickr - pictures
- Listal - Books, DVDs, Music and Games
- Rojo - RSS_(file_format) and Atom_(standard) web_syndication and weblog entries.
- Simpy - social bookmarks & notes
- Technorati - weblog entries, weblogs themselves