Difference between XSDL and DTD ?

XSDL stands for Extensible Schema Definition Language.  Schema  define the structure and content of the message that you will be processing. Schema   can   be used  to validate that incoming message are properly formatted or not.

DTD stands for Document Type Definition.

In a DTD,

(i) sets of elements and their attributes are defined;

(ii) the names that are used as tags are assigned; and

(iii) the element relationships or transaction is defined.
If a DTD is used then programs can validate the transaction’s structure. The best known example of a DTD is HTML, which is defined by a SGML-DTD describing the structure of HTML documents.