What is XML?
XML = eXtensible Markup Language
XML is a subset of SGML -- Standard Generalized Markup Language, but unlike the latter, XML is specifically designed for the web
XML is used to define logical document structure
XML is not backward compatible with HTML, but the two coexist quite elegantly
XML can be thought of as an ASCII version of a database
XML can be used as an object IDL to access databases or any objects and so give web HTML templates to access web-linked databases / backend services
XML can be used as inter object data (serialization) format for any object model
XHTML is HTML4 written in XML syntax