• What is DTD?
  The Document Type Definition (DTD) defines the valid syntax of a class of XML documents. That is, it lists a number of element names, which elements can appear in combination with other ones, what attributes are available for each element type, etc. A DTD uses a different syntax (namely Extended Backus Naur Form) from that used by XML documents.



• What is Schema?
  A schema - in an XML parlance, is used to describe the possible data content of a document in a very rigorous and formal way. Much like a contract or a treaty is used to spell out the terms of a business or formal social agreement, a schema is used to codify the intention to use a particular set of mark-up tag names, and combine the tags in a particular order.
  Schema specification links:
  

  XML Schema Part 0: Primer
  XML Schema Part 1: Structures
  XML Schema Part 2: Datatypes
  


Be sure to check out A Quick Guide to XML Schema.


• What is Namespace?
  XML Namespaces provide a simple method for qualifying element and attribute names used in Extensible Markup Language documents by associating them with namespaces identified by URI references. XML Namespaces are the solution to the problem of ambiguity and name collisions.
  
  Official Namespace Specification is here.

We'll be adding new sections here soon. Till then enjoy the following articles.

• Read Chapter 6: XML Schemas from the book Beginning XML, 2nd Edition.
  
• DTDs in the realm of XML


• Document Type Definition FAQ


• XML, Validation, and Extra Cheese


• DTD and DOM


• ABC's of Schema


• Schemas: Tapping the Potential Within


• Using Schema and Serialization to Leverage Business Logic


• XML Schema Developer's Guide


• Schemas Revisited


• Schema.net


• Why Use XML Namespaces


• XML Namespaces FAQ


• Understanding XML Namespaces