XML is short for - Extensible Markup Language (XML) is a general-purpose markup language.
Its primary purpose is to facilitate the sharing of data across different information systems, particularly via the Internet. It allows for the separation of content from layout and delivery mechanism. XML document do not contain information how to display the data, but using the CSS or XSL it can be done.
It is very important to understand that XML was created to structure, store and to send information. Using XML it's possible to example separate data from XHTML/HTML