XML is a three-letter-acronym meaning Extensible Markup Language.
XML is a specification managed by the World Wide Web consortium (W3C) which describes a general-purpose approach for creating data-files that can be exchanged between computer systems. This Extensible language allows developers to create custom tags that describe data-elements separate from their format and presentation. That helps developers of different applications exchange data between applications.
Read more about XML at the following pages:
Microsoft XML Developer Center