XML (Extensible Markup Language) is a markup language used to describe data, the XML based version of HTML is called XHTML, but it has few, if any advantages, to just plain HTML.
Many sites using XHTML has traditionally been incorrectly served using a text/html mime type, while XML requires a application/xhtml+xml mime type.
XML is also used for RSS and Atom feeds, as well as for Sitemaps. Feeds, such as RSS and Atom are typically generated by the CMS system, users can then subscribe to these feeds, to get notified when you post or write new content for your website. Sadly, recent versions of Google Chrome does not support news feeds.
XML resources should be served with the application/xhtml+xml mime type. See also: XHTML and Mime Types