Extensible Markup Language (XML) is a simple, very flexible text format derived from SGML. Originally designed to meet the challenges of large-scale electronic publishing, XML now plays an increasingly important role in the exchange of a wide variety of data on the Web and elsewhere.
W3C XML Specification
The World Wide Web Consortium (W3C) XML Specification is developed and maintained by the W3C XML Core Working Group.
W3 Schools
At W3 Schools you will find a collection of free HTML, CSS, JavaScript, DHTML, XML, XHTML, WAP, ASP, SQL tutorials with lots of working examples and source code.
Top XML
At TopXML you will find XML tools, tutorials and many other resources.
MSXML
Microsoft XML Core Services (MSXML) allows you to use DXL to build XML-based applications that provide a high degree of interoperability with other applications that adhere to the XML 1.0 standard.
Download MSXML 4.0 SP2
MSDN Library
Read about MSXML at the MSDN Library.
MSXML 4.0 Overview
Read an independent overview of MSXML 4.0.