What is the relation B/W XHTML and DTD?
Document Type Definition (DTD) is a set of markup declarations that define a document type for SGML-family markup languages (SGML, XML, HTML). A DTD is a kind of XML schema.
DTDs use a terse formal syntax that declares precisely which elements and references may appear where in the document of the particular type, and what the elements’ contents and attributes are. DTDs also declare entities which may be used in the instance document.
So, the DTD for XHTML describes for XHTML which elements and attributes are allowed. Everything that doesn’t appear in the DTD is not allowed (for example placing an <a> directly in an <ul>).
If you are thinking that changing the DTD in the doctype declaration to a URL with your own DTD so you extend HTML with your own tags and/or attributes and let it still validate, I have to disappoint you, that doesn’t work (I’ve tried it).
This question is covered in the FAQ that you were pointed to yesterday.