From http://weblogs.asp.net/rchartier/archive/2006/03/21/440782.aspx
DTD's are not namespace aware.
DTD's have #define, #include, and #ifdef -- or, less C-oriented, the ability to define shorthand abbreviations, external
content, and some conditional parsing.
A DTD describes the entire XML document (even if it leaves "holes"); a schema can define portions.
XSD has a type system.
XSD has a much richer language for describing what element or attribute content "looks like." This is related to the type
system.
You can put a DTD inline into an XML document, you cannot do this with XSD. This means DTD's are more secure (you only
have to protect one bytestream -- the xml/dtd -- and not
multiple).
The official definition of "valid XML" requires a DTD. Since this may be impractical, if not impossible, you often have to
settle for schema-valid, which is not quite the same.
For my part, it's pretty straightforward to write a validator for some XML if you have an XSD. I haven't seen this with a DTD, although I'm sure it exists.
