Extensible Markup Language (XML) is the universal language for data on the Web. It gives developers the power to deliver structured data from a wide variety of applications to the desktop for local computation and presentation. XML allows the creation of unique data formats for specific applications.
XML is all about metadata and the idea that certain groups of people have similar needs for describing and organising the data they use. Like HTML, XML is a set of tags and declarations - but rather than being concerned with formatting information on a page, XML focuses on providing information about the data itself and how it relates to other data.
It is likewise an ideal format for server-to-server transfer of structured data.
If you want to learn more, you can read the article: XML - What's in it for us?