The eXtensible Markup Language (XML) is a simplified form of SGML and is a meta-language for creating markup languages. Examples of markup languages are:
the elements and attributes of a Java web application's web.xml file, the elements and attributes of a
Struts configuration file, SOAP, WSDL, FpML, DocBook, BPML, etc. These are commercial examples of XML-based markup languages, there are many, many others. And many proprietary ones that are specific to internal apps at a specific company,
Check out the
http://xml.coverpages.org/ website for a few resouces.
Aside, the actual languages are used to describe data, not transform it. There are transformation tools and concepts that work with XML-based data to execute data transformation. However, this is what is "done" with data in XML-format, not the purpose of XML itself. There are many other ways it is used, the Java web apps web.xml file is a good example.
Good luck!