XML -Extensible Markup Language, the universal format for structured documents and data on the Web.
HTML - HyperText Markup Language, the lingua franca for publishing hypertext on the World Wide Web.
DOM - Document Object Model represents a tree view of the XML document.
DTD - Document Type Definition defines the types of tags can be used in and XML document
HTML began as a reduced nonproprietary subset of SGML that web browsers use to reproduce an authors information in a dependable form. It is based on tag pairs (<font> </font>) that tell a web browser how to format information between the tags for display. XML is a generalization created on top of HTML which allows creation of new tags usually in a DTD documents or in the document itself.