Brief Description
The Extensible Markup Language (XML) is a data format for structured document interchange on the Web supported by the World Wide Web Consortium (W3C). XML is a subset of the Standard Generalized Markup Language ( SGML ). For a good introduction to XML you can look here
.
XML document's type is found in a DTD (Document Type Definition). In this files there is a definition of the XML document structure (What data will there be in the document, where and how).
As XML don't include directly style information, those information are stored in a XSL file (extensible Style Language). This XSL permits to format the data included in the XML file, even complex formatting using scripting.
XML Links
www.xml.com
is good site for news, specifications and DTD
http://www.w3.org/XML/
is the entry page for XML at the W3C site.
The SGML/XML Web Page
A very good entry point for XML and related resources on the Web.
XML repository 
XML FAQ 
DTDs and XSL Links
Microsoft's XML site
presents many XML and XSL samples
http://www.schema.net/
presents most of the currently available DTD's
http://www.xmlsoftware.com/
lists the softwares related to XML (editors, parsers, browser)
http://www.xmlinfo.com/
gives general information about XML and related news
Newsgroups
comp.text.xml
and comp.text.sgml
News group of general interest for XML.
The XML-L
mailing list.
Microsoft.public.xml
This news group is devoted to XML and related Microsoft products like IE5 or msxml ( XML parser ).
Remark
RadialSoft interest in XML is e-business and CADCAM data exchange on the Web.
XML