What is XML?

XML is a standard by the W3C consortium prescribing how to create markup languages.

It is therefore a metalanguage.

It is ideologically based on older standards (SGML Structure Generalized Markup Language) -- XML can be seen as almost a subset of SGML.

There are several other standards closely related to XML, such as XML Namespaces, XInclude, XML Base, XML Infoset.

These standards together with others (XSLT, XSL-FO, XHTML, CSS ...) form a "family" of XML standards.