eXtensible Markup Language (XML) is a universal format, maintained by the W3C, used for representation and transfer of structured data on the web or between different applications.
The language uses a structured representation by allowing users to create custom defined tags according to XML Document Type Definition (DTD) standards. The structure of an XML document can be represented in the form of a tree known as a Document Object Model (DOM).
Source: eXtensible Markup Language (XML)