DTD (or Document Type Definition) is one of the original schema languages to come out with the XML 1.0 standards. However, in the .NET world it is rather unpopular, owing perhaps to it's complexity, unxml-like format and Microsoft's embrace of XSD. All the same, if you happen to run into a DTD schema document, which I did, there is a great online convertor available at http://www.hitsw.com/xml_utilites/ which can covert XSDs from DTD or XML Documents to DTD. It also supports generating XML schemas from XML Documents, just like Microsoft's Xsd.exe tool available with Visual Studio.
Also worth mentioning is the Xsd2Code Visual Studio plugin available at Codeplex. As the name suggests, it converts XML schema files to C# or VB.NET code, again like Microsoft's Xsd.exe tool. However, what is nice is it also adds explicit support for serialization and deserialization, nullable types, INotifyPropertyChanged and most importantly generic collections. Cool stuff.