XML: Short for "Extensible Markup Language". A flexible way to create common information formats that shares both the format and the data on the World Wide Web, intranets, and elsewhere. XML is "extensible" because, unlike HTML, the markup symbols are unlimited and self-defining.
XML is actually a simpler and easier-to-use subset of the Standard Generalized Markup Language (SGML), the standard for creating a document structure. It is expected for XML and HTML to be used together in many web applications.