XML, which stands for Extensible Markup Language, is a language similar to HTML used for creating arbitrarily structured documents and web pages. The language is a filtered version of SGML, Standard Generalized Mark-Up Language, but more suited for web delivery with fewer overheads common with SGML. Unlike HTML, XML specifies no pre-defined tags. Instead, the semantics of an XML document are defined by the applications that process them.

XML files are written in ASCII text, so the information contained within an XML document is exchangeable between otherwise incompatible systems. Although commonly associated with the Internet, XML is not necessarily limited to such. Since it is a text file, it can be viewed in any text editor. In order to view an XML document's structure you'll need an XML parser. This parser will read the XML document and display it in an organized fashion creating collapsing and expanding branches. If the XML document is to be viewed over the Internet, the document must first be converted into HTML through XSL.