XML is not really a language in itself, but a set of rules for creating languages. There is an XML language called CML, designed for storing and displaying information about chemicals. There is an XML language called MathML, designed for storing and displaying mathematical formulae. And there are hundreds of other XML languages; some very specialised, some more general.
An XML language (which is technically called an XML application) is a format for storing knowledge. XML documents can be easily manipulated using software tools, and are similar in many respects to databases. Information from within an XML document can be selectively extracted (using a query language called XPath), which makes it possible to re-use and merge information from different sources.
XML technologies are all open-source; they belong to humanity. The World Wide Web Consortium (W3C) coordinates the direction and development of the underlying XML technologies.
An XML application is defined or specified in a schema document. (Schemas used to be known as Document Type Definitions, or DTDs.)