Ininformation systems, atag is akeyword or term assigned to a piece of information (such as anInternet bookmark,multimedia, databaserecord, orcomputer file). This kind ofmetadata helps describe an item and allows it to be found again by browsing or searching.[1] Tags are generally chosen informally and personally by the item's creator or by its viewer, depending on the system, although they may also be chosen from acontrolled vocabulary.[2]: 68
Tagging was popularized bywebsites associated withWeb 2.0 and is an important feature of many Web 2.0 services.[2][3] It is now also part of otherdatabase systems,desktop applications, andoperating systems.[4]
People use tags to aidclassification, mark ownership, noteboundaries, and indicateonline identity. Tags may take the form of words, images, or other identifying marks. An analogous example of tags in the physical world ismuseum object tagging. People were using textualkeywords toclassify information and objects long before computers. Computer basedsearch algorithms made the use of such keywords a rapid way of exploring records.
Tagging gained popularity due to the growth ofsocial bookmarking,image sharing, andsocial networking websites.[2] These sites allow users to create and manage labels (or "tags") that categorize content using simple keywords. Websites that include tags often display collections of tags astag clouds,[a] as do some desktop applications.[b] On websites that aggregate the tags of all users, an individual user's tags can be useful both to them and to the larger community of the website's users.
Tagging systems have sometimes been classified into two kinds:top-down andbottom-up.[3]: 142 [4]: 24 Top-downtaxonomies are created by an authorized group of designers (sometimes in the form of acontrolled vocabulary), whereas bottom-up taxonomies (calledfolksonomies) are created by all users.[3]: 142 This definition of "top down" and "bottom up" should not be confused with the distinction between asingle hierarchicaltree structure (in which there is one correct way to classify each item) versusmultiple non-hierarchicalsets (in which there are multiple ways to classify an item); the structure of both top-down and bottom-up taxonomies may be either hierarchical, non-hierarchical, or a combination of both.[3]: 142–143 Some researchers and applications have experimented with combining hierarchical and non-hierarchical tagging to aid in information retrieval.[7][8][9] Others are combining top-down and bottom-up tagging,[10] including in some large library catalogs (OPACs) such asWorldCat.[11][12]: 74 [13][14]
When tags or other taxonomies have further properties (orsemantics) such asrelationships andattributes, they constitute anontology.[3]: 56–62
In folder system a file cannot exist in two or more folders so tag system has been thought more convenient. But transitioning to tag system requires awareness of difference between properties of two systems. In folder system the information of classification is put outside of the file and we can change folder at once. In tag system the information of classification is put inside the file so changing its tag means changing the file and it needs to be saved again and takes time.
Metadata tags as described in this article should not be confused with the use of the word "tag" in some software to refer to an automatically generatedcross-reference; examples of the latter aretags tables inEmacs[15] andsmart tags inMicrosoft Office.[16]
The use of keywords as part of an identification and classification system long predates computers.Paper data storage devices, notablyedge-notched cards, that permitted classification and sorting by multiple criteria were already in use prior to the twentieth century, andfaceted classification has been used by libraries since the 1930s.
In the late 1970s and early 1980s,Emacs, the text editor forUnix systems, offered a companion software program calledTags that could automatically build a table of cross-references called atags table that Emacs could use to jump between afunction call and that function's definition.[17] This use of the word "tag" did not refer to metadata tags, but was an early use of the word "tag" in software to refer to aword index.
Online databases and early websites deployed keyword tags as a way for publishers to help users find content. In the early days of theWorld Wide Web, thekeywordsmeta element was used byweb designers to tellweb search engines what the web page was about, but these keywords were only visible in a web page'ssource code and were not modifiable by users.
In 1997, the collaborative portal "A Description of the Equator and Some ØtherLands" produced bydocumenta X, Germany, used thefolksonomic termTag for its co-authors and guest authors on its Upload page.[18] In "The Equator" the termTag for user-input was described as anabstract literal or keyword to aid the user. However, users defined singularTags, and did not shareTags at that point.
In 2003, thesocial bookmarking websiteDelicious provided a way for its users to add "tags" to their bookmarks (as a way to help find them later);[2]: 162 Delicious also provided browseable aggregated views of the bookmarks of all users featuring a particular tag.[19] Within a couple of years, thephoto sharing websiteFlickr allowed its users to add their own text tags to each of their pictures, constructing flexible and easy metadata that made the pictures highly searchable.[20] The success of Flickr and the influence of Delicious popularized the concept,[21] and othersocial software websites—such asYouTube,Technorati, andLast.fm—also implemented tagging.[22] In 2005, theAtom web syndication standard provided a "category" element for inserting subject categories intoweb feeds, and in 2007Tim Bray proposed a "tag"URN.[23]
Many systems (and other webcontent management systems) allow authors to add free-form tags to a post, along with (or instead of) placing the post into a predetermined category.[a] For example, a post may display that it has been tagged withbaseball andtickets. Each of those tags is usually aweb link leading to an index page listing all of the posts associated with that tag. The blog may have a sidebar listing all the tags in use on that blog, with each tag leading to an index page. To reclassify a post, an author edits its list of tags. All connections between posts are automatically tracked and updated by the blog software; there is no need to relocate the page within a complex hierarchy of categories.
Somedesktop applications andweb applications feature their own tagging systems, such as email tagging inGmail andMozilla Thunderbird,[12]: 73 bookmark tagging inFirefox,[24] audio tagging iniTunes orWinamp, and photo tagging in various applications.[25] Some of these applications display collections of tags astag clouds.[b]
There are various systems for applying tags to the files in a computer'sfile system.
InApple'sMacSystem 7, released in 1991, users could assign one ofseven editable colored labels (with editable names such as "Essential", "Hot", and "In Progress") to each file and folder.[26] In later iterations of the Mac operating system ever sinceOS X 10.9 was released in 2013, users could assign multiple arbitrary tags asextended file attributes to any file or folder,[27] and before that time theopen-source OpenMeta standard provided similar tagging functionality forMac OS X.[28]
Severalsemantic file systems that implement tags are available for theLinux kernel, includingTagsistant.[29]
Microsoft Windows allows users to set tags only onMicrosoft Office documents and some kinds of picture files.[30]
Cross-platform file tagging standards includeExtensible Metadata Platform (XMP), anISO standard for embedding metadata into popular image, video and document file formats, such asJPEG andPDF, without breaking their readability by applications that do not support XMP.[31] XMP largely supersedes the earlierIPTC Information Interchange Model.Exif is a standard that specifies the image and audiofile formats used bydigital cameras, including some metadata tags.[32]TagSpaces is an open-source cross-platform application for tagging files; it inserts tags into thefilename.[33]
Anofficial tag is a keyword adopted by events and conferences for participants to use in their web publications, such as blog entries, photos of the event, and presentation slides.[34] Search engines can then index them to make relevant materials related to the event searchable in a uniform way. In this case, the tag is part of acontrolled vocabulary.
A researcher may work with a large collection of items (e.g. press quotes, a bibliography, images) in digital form. If he/she wishes to associate each with a small number of themes (e.g. to chapters of a book, or to sub-themes of the overall subject), then a group of tags for these themes can be attached to each of the items in the larger collection.[35] In this way, freeformclassification allows the author to manage what would otherwise be unwieldy amounts of information.[36]
Atriple tag ormachine tag uses a specialsyntax to define extrasemantic information about the tag, making it easier or more meaningful for interpretation by a computer program.[37] Triple tags comprise three parts: anamespace, apredicate, and a value. For example,geo:long=50.123456 is a tag for the geographicallongitude coordinate whose value is 50.123456. This triple structure is similar to theResource Description Framework model for information.
The triple tag format was first devised for geolicious in November 2004,[38] to mapDelicious bookmarks, and gained wider acceptance after its adoption by Mappr and GeoBloggers to mapFlickr photos.[39] In January 2007, Aaron Straup Cope at Flickr introduced the termmachine tag as an alternative name for the triple tag, adding some questions and answers on purpose, syntax, and use.[40]
Specialized metadata for geographical identification is known asgeotagging; machine tags are also used for other purposes, such as identifying photos taken at a specific event or naming species usingbinomial nomenclature.[41]
A hashtag is a kind of metadata tag marked by the prefix#, sometimes known as a "hash" symbol. This form of tagging is used onmicroblogging andsocial networking services such asTwitter,Facebook,Google+,VK andInstagram. The hash is used to distinguish tag text, as distinct, from other text in the post.
Aknowledge tag is a type ofmeta-information that describes or defines some aspect of a piece of information (such as adocument,digital image,database table, orweb page).[42] Knowledge tags are more than traditional non-hierarchicalkeywords or terms; they are a type ofmetadata that captures knowledge in the form of descriptions, categorizations, classifications,semantics, comments, notes, annotations,hyperdata,hyperlinks, or references that are collected in tag profiles (a kind ofontology).[42] These tag profiles reference an information resource that resides in a distributed, and often heterogeneous, storage repository.[42]
Knowledge tags are part of aknowledge management discipline that leveragesEnterprise 2.0 methodologies for users to capture insights, expertise, attributes, dependencies, or relationships associated with a data resource.[3]: 251 [43] Different kinds of knowledge can be captured in knowledge tags, including factual knowledge (that found in books and data), conceptual knowledge (found in perspectives and concepts), expectational knowledge (needed to make judgments and hypothesis), and methodological knowledge (derived from reasoning and strategies).[43] These forms ofknowledge often exist outside the data itself and are derived from personal experience, insight, or expertise. Knowledge tags are considered an expansion of the information itself that adds additional value, context, and meaning to the information. Knowledge tags are valuable for preserving organizational intelligence that is often lost due toturnover, for sharing knowledge stored in the minds of individuals that is typically isolated and unharnessed by the organization, and for connecting knowledge that is often lost or disconnected from an information resource.[44]
In a typical tagging system, there is no explicit information about the meaning orsemantics of each tag, and a user can apply new tags to an item as easily as applying older tags.[2] Hierarchical classification systems can be slow to change, and are rooted in the culture and era that created them; in contrast, the flexibility of tagging allows users to classify their collections of items in the ways that they find useful, but the personalized variety of terms can present challenges when searching and browsing.
When users can freely choose tags (creating afolksonomy, as opposed to selecting terms from acontrolled vocabulary), the resulting metadata can includehomonyms (the same tags used with different meanings) andsynonyms (multiple tags for the same concept), which may lead to inappropriate connections between items and inefficient searches for information about a subject.[45] For example, the tag "orange" may refer to thefruit or thecolor, and items related to a version of theLinux kernel may be tagged "Linux", "kernel", "Penguin", "software", or a variety of other terms. Users can also choose tags that are differentinflections of words (such as singular and plural),[46] which can contribute to navigation difficulties if the system does not includestemming of tags when searching or browsing. Larger-scale folksonomies address some of the problems of tagging, in that users of tagging systems tend to notice the current use of "tag terms" within these systems, and thus use existing tags in order to easily form connections to related items. In this way, folksonomies may collectively develop a partial set of tagging conventions.
Despite the apparent lack of control, research has shown that a simple form of shared vocabulary emerges in social bookmarking systems. Collaborative tagging exhibits a form ofcomplex systems dynamics (orself-organizing dynamics).[47] Thus, even if no central controlled vocabulary constrains the actions of individual users, the distribution of tags converges over time to stablepower law distributions.[47] Once such stable distributions form, simplefolksonomic vocabularies can be extracted by examining thecorrelations that form between different tags. In addition, research has suggested that it is easier formachine learning algorithms to learn tag semantics when users tag "verbosely"—when they annotate resources with a wealth of freely associated, descriptive keywords.[48]
Tagging systems open to the public are also open to tag spam, in which people apply an excessive number of tags or unrelated tags to an item (such as aYouTube video) in order to attract viewers. This abuse can be mitigated using human or statistical identification of spam items.[49] The number of tags allowed may also be limited to reduce spam.
Some tagging systems provide a singletext box to enter tags, so to be able totokenize the string, aseparator must be used. Two popular separators are thespace character and thecomma. To enable the use of separators in the tags, a system may allow for higher-level separators (such asquotation marks) orescape characters. Systems can avoid the use of separators by allowing only one tag to be added to each inputwidget at a time, although this makes adding multiple tags more time-consuming.
A syntax for use withinHTML is to use therel-tagmicroformat which uses therel attribute with value "tag" (i.e.,rel="tag") to indicate that the linked-to page acts as a tag for the current context.[50]
As with all the other options here, meta data can be added to individual files to help improve their find-ability, and uniquely the tag cloud field within Leap's interface allows you to quickly drill down to individually labelled files without fuss.
Calling a function defined in one compilation unit from within another is analogous to cross references in large hypertext documents. By using tags tables, the Emacs environment enables the user to turn program source code into powerful hypertext documents.
You can turn on smart tags for a field to make it easier to cross-reference data between the Access database and Microsoft Outlook (or another personal information and e-mail program) and the Web.
EMACS is an M.I.T. display editor designed to be 'extensible, customizable, and self-documenting' [...] Another interesting facility for program editing is the TAGS package. The separate program TAGS builds a TAGS table containing the file name and position in that file in which each application program function is defined. This table is loaded into EMACS; specifying the command Meta,function name causes EMACS to select the appropriate file and go to the proper function definition within that file.
Tags were not in the initial version of Flickr. Stewart Butterfield wanted to add them. He liked the way they worked on del.icio.us, the social bookmarking application. We added very simple tagging functionality, so you could tag your photos, and then look at all your photos with a particular tag, or any one person's photos with a particular tag. Soon thereafter, users started telling us that what was really interesting about tagging was not just how you've tagged your photos, but how the whole Flickr community has been tagging photos. So we started seeing a lot of requests from users to be able to see a global view of the tagscape.