XML 
Extensible Markup Language 
A W3C specification developed from SGML used for structuring content in Web pages and other documents. It uses a tag structure similar to that used in HTML, but the tags are used to define data and content elements rather than describe how they should look on a Web page. Developers can create any number of tags, and they can specify how the content or data coded with those tags appears on screen, or interacts in a variety of ways. This system allows documents to essentially function like databases.