Tag technology involves pieces of data being labeled in a standardized fashion. The most well-known use of this is in HTML, which tags information on webpages to control its layout and appearance. Other web-based tagging systems include CSS and XML.
Hypertext Mark-Up Language, or HTML, is likely the most used form of tag technology. It is the basis of how web pages work and why they are different than simple text documents. HTML coding involves sections of text being enclosed in a tag. The tag begins with the sign. Tags are used in pairs, with the second repeating the tag but preceding it with a / sign to show that the section of text to be tagged has ended.
Computer software which handles HTML, such as a web browser, knows that text tagged in this way is information about the content of the page, rather than text to be displayed to the reader. Most HTML tags relate to the visual formatting of the text, such as it being shown in bold, italics or underlined. Some tags also describe its function, most notably when text links to another web page.
A more advanced form of tag technology used on websites is Cascading Style Sheets or CSS. When writing a page in CSS, the web designer will use tags to indicate the type of text a section is, for example, a sub-heading or a long quotation, but will not specifically state how this text should appear. Instead this information will be contained in a separate document known as a stylesheet. One advantage of this is that updating the stylesheet can instantly change the appearance of all text with a particular tag across an entire website rather than each example having to be individually updates. Another advantage is that the person viewing the site can use their own stylesheet and have more control about how the page appears.
Extensible Markup Language, or XML, takes a different approach to tag technology. Instead of being limited to determining how text appears, XML tags can describe the type of information contained in that text. This is similar to the use of fields in a database and means XML-based webpages and documents are much easier to integrate into database tools. Unlike HTML, which uses a standardized set of tags, XML allows customizable tags. For example, if a website of city guides used an XML tag named "busroutes" to mark up the section on each page listing local bus routes, it would be simple to automatically produce a new page which brought together bus route information from all cities.