XML is a great way for systems of different breed to communicate in a standard way. For example one system in your company is UNIX and the other is Windows. These systems need to send/receive organized data to each other. XML allows this with the use of server side technology. In many cases XML or JSON will be used on the client as well with a language such as JavaScript.

"The Extensible Markup Language (XML) is a general-purpose markup language. It is classified as an extensible language because it allows its users to define their own tags. Its primary purpose is to facilitate the sharing of structured data across different information systems, particularly via the Internet." - wikipedia.com

Here is an example of the XML document that contains contact data with a one-to-many relationship on the contact to many phone numbers.

Works Cited
Wikipedia XML. (n.d.). Retrieved from XML:

http://en.wikipedia.org/wiki/XML