XML (Extensible Markup Language)

Extensible Markup Language (XML) is a universal markup language and file format, used to represent, store and transmit structured data on the web or between different applications.
XML defines a set of rules for encoding documents in a format that can be read by both humans and computers. These rules are maintained by the World Wide Web Consortium. XML-based standards provide simplicitiy, portability, and platform independence for programming languages.

Markup Language constructs are known as 'tags'; within XML, the user is free to define their own tags — hence the term 'extensible.'