hi friends.
we will start from scratch.. at first XML stands foe eXtensible Markup Language.

eXtensible -> Where u can define ur own tags to create the data file.
Markup -> The language which uses tags to store data.

markup languages are derived from SGML as our friends said above. SGML was a very vast lang which provides both defined tags and manual user tags. So it was very difficult to work with SGML so ppl came with solutiion like HTML (All tags predefined) XML (None of tags are defined u have create ur own tags)

there are few rules for XML.
1. Well formed
2. Valid

Well formed means all the tags should be properly nested
Valid means the file is composed according to DTD (document type definition) or XSD

XML is all about data. Porting data between incompatible system is the best feature of XML.

U can also display XML file as HTML, TEXT, PDF (using FOP) for this we need another X called XSL and XSl FO which internally usase XSLT(An Engine)

U can query the XML file using XPATH or XQUERY

U can give links using XLINK.

For parsing a XML file we either use (SAX or DOM) read on net about them.

The future of XML is very bright

well one thing wanna say XML is not a replacement of HTML.

Happy X 'ing