XML (Extensible Markup Language) is essentially a languae for defining languages.

Markup language describes how the content of documents should be interpreted.

XML is an easier-to-use subset of SGML (Standard Generalized Markup Language). HTML is an application of SGML and that is probably the only application of SGML that is internationally widely accepted and supported.


Free, but very simple XML editor: XML Notepad

Rich resrouce: XML.org, W3C