What is XML
As I see XML ... Its a human readable file which has elements and attributes to transfer data in a objectified way. Elements can be considered as objects and attributes as object's variables. It is easy to read. eg.
<!-- comments follows -->
<element2 attribute1="1234" attribute2="234"> Text to be put up</element2>
XML has many more kind of data which it takes esp text. As compared to RDBMS ( which has columns and rows as in excel sheet to store data. ) XML is more powerfull. XML helps developer to define his own rules ( DTD, XSD ) to generate apply datastructure as per the need of the system.
XML has validation rules in form of file ( DTD or XSD ). DTD has loose validation rules and the XSD has strict typed rules ( including regular expressions )
XML are mostly used in data transfer accross the modules, applications, and in the configuration of the application. Its extensively used in building scripts esp apache-ant.
I will add more to it as I read on.