Peruse the official, W3C specification for Extensible Markup Language, with added notes and commentary.

Spec for a data model for describing information organization structures (metadata) for collections of networked information using XML.

Provides an overview of meta content framework using the XML programming language.

Here you will find the full HTML version of the complete description of the proposed standard for XML.

Read a 1997 draft of the XML spec, including links, presented by the W3C. XML is a dialect of SGML designed for ease of use on the Web.

World Wide Web Consortium offers this handy table outlining status, syntax and examples of protocols including XML-RPC, ICE and WDDX.