Extensible Markup Language (XML)

Extensible Markup Language (XML) is a language used for formatting text and storing information about content (called metadata). XML presents information in a standard form that can be used to communicate between different types of computer systems connected to the Internet. XML emerged in the 1990s primarily to facilitate electronic publishing over the Internet. XML became a standard in 1998, and has been used extensively since.

Most Web pages contain a mixture of text, images, files, video, music and other types of data. XML provides a way to include metadata, which describes the content on the page, but it does not enforce rules on how to display the page.

Put simply, XML is a way to represent data in an ordered and hierarchical format, so that any application, not just Web browsers, can understand, format and make use of this information for various purposes.

Many applications and standards are derived from XML. Examples are Really Simple Syndication (RSS), used for news feeds; Extensible Hypertext Markup Language (XHTML), used for creating, formatting and displaying Web pages; Scalable Vector Graphics, used for displaying special "3D within 2D" images; and GraphML, used for displaying graphs or plots. XML is very flexible in that it can easily be expanded.

XML documents can be created in any standard text editor, even in the Notepad found in MS Windows. XML simply provides a syntactic foundation for how the document should be defined, but the actual structure of the document, the layout, the look, the meaning, etc., can all be customized under something called schemas. There might be a number of XML elements which can be combined in different ways to produce completely different structured documents. The schemas provide the "rules" for how the document should be created from these elements. For example, using XML schemas, the same content of a user manual can be reused and easily reformatted into the Help section of a Web site.

Since XML is mostly used as a data description language, most Web browsers will just display XML files in their raw form. Style sheet languages like Cascade Style Sheet (CSS) or the Extensible Stylesheet Language (XSL) are required to "stylize" this data so that browsers can display it. The same XML file may appear very differently according to the CSS/XSL style sheet used to display it.