XML is the general form of HTML; with it, a programmer can create his/her own tags. Read these slides and be sure you understand the XML Elements.
Namespaces and XML DTD are components of an XML schema, a set of rules governing the ways in which a Web Server will parse user-specific XML documents for display. Read this chapter.
Read this webpage for an overview of the purpose, function, and format of XML namespaces.
Read this webpage.
Complete this review.