Extensible Markup Language is a metalanguage that enables definiton of customized markup languages.
Wikipedia article: xml
XML is both human and machine readable. That said, most humans will take a look at it and want to close their eyes – but with a little bit of knowledge about extensible markup language it’s possible to interpret relatively easily.
Here is a sample of xml in this case used for OPML (Outline Processor Markup Language). “OPML is an XML format for outlines (defined as "a tree, where each node contains a set of named attributes with string values.” – Wikipedia OPML
1: <?xml version="1.0" encoding="UTF-8"?>
2: <opml version="1.0">
7: <outline title="Ballarat Library Chick" type="RSS" xmlURL="http://ballaratlibrarychick.blogspot.com/feeds/posts/default?alt=rss" htmlURL="http://ballaratlibrarychick.blogspot.com.au/" />
8: <outline title="When the moon shines" type="rss" xmlUrl="http://whenthemoonshines.com/feed/" htmlUrl="http://whenthemoonshines.com"/>
OPML is used to migrate a list of RSS feeds from one feed reader to another. This sample is a cut down version of the OPML file I created of all the #blogjune rss feeds.