XML is a W3C Recommendations. It stands for Extensible Markup Language . It is a markup language much like HTML used to describe data. In XML, tags are not predefined. A user defines his own tags and XML document structure like Document Type Definition (DTD) , XML Schema to describe the data. Hence it is self-descriptive too.
Please read xml Introduction at http://www.roseindia.net/xml/xml-introduction.shtml
Read more tutorials on xml at http://www.roseindia.net/xml/