Hi Amal
At it's most basic, XML is a text-based storage mechanism for data. XML allows you to define your own languages (called vocabularies) to describe data. The focus of my book is using XML to power Web applications and the role of XML is explained in detail in Chapter 1.
XML is one of a family of related recommendations that cover areas such as how to define XML languages, how to display XML content, how to find content inside XML documents and how to link to XML content.
XML isn't a programming language so, in Web applications, you need to work with it using client or server-side scripting.
I hope that helps. You can read a more detailed explanation at
http://www.xml.com/pub/a/98/10/guide0.html. Cheers
Sas Jacobs