XML in Action

HTML (Hypertext Markup Language) has been called the foundation of the World Wide Web. It provides a standardized way to create pages of formatted information that can be delivered to an ever-increasing global audience by means of the Internet. In a very real way, HTML—combined with HTTP (Hypertext Transport Protocol)—has revolutionized how people send and receive information. But HTML was designed primarily for data display. As a result, HTML focuses almost entirely on how information looks, not on what the information is or how it is structured. This is where Extensible Markup Language (XML) comes in.

XML is an open, text-based markup language that provides structural and semantic information to data. This "data about data," or metadata, provides additional meaning and context to the application using the data and allows for a new level of management and manipulation of Web-based information. XML, a subset of the popular Standard Generalized Markup Language (SGML), has been optimized for the Web. This helps make XML a powerful, standards-based complement to HTML that could be as impotant to the future of information delivery on the Web as HTML was to its beginning.

XML is intended to be used by content creators as well as by programmers. Since XML is text-based, it can be read and worked with easily in relatively nontechnical situations, but its ability to organize, describe, and structure data also makes it ideal for use in highly technical applications. XML thus provides common ground for creating structured data and making it available for manipulation and display.

Although XML is often called a language, it is, strictly speaking, a metalanguage. This means that XML can be used to create other languages. Thereby, vocabularies, which are language applications of XML, can be created to address targeted needs or solve specific problems. Some examples of XML vocabularies include:

* XSL A style sheet_based formatting language that can take XML data and produce a wide range of output results.
* XLL An extended linking specification that takes the hyperlinking mechanism of HTML to new levels.
* SMIL A standardized multimedia authoring language that allows sophisticated multimedia authoring on the Web.
* XSL Patterns An XML query language that provides advanced searching capabilities on XML data.


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