XML (eXtensible Markup Language) is a meta-language; that is, it is a language in which other languages are created. In XML, data is "marked up" with tags, similar to HTML tags. In fact, there is an XML version of HTML, called XHTML. XHTML has the same tags as HTML, but it follows the syntax rules of XML.
XML is used to store data or information; this data might be intended to be read by people or by machines. It can be highly structured data such as data typically stored in databases or spreadsheets, or loosely structured data, such as data stored in letters or manuals.