XSL stands for Extensible Stylesheet Language. It is a language for expressing stylesheets. These stylesheets are like CSS which describes how to display an XML document of a given type.
1) DTD stands for Document Type Definition whereas XSD stands for XML Schema Definition.
2) DTD does not support namespaces. It has its own set of keywords to define a schema whereas XSD uses its own set of namespaces and elements to define the schema.
3) DTD is the predecessor of XML schema whereas XML schema is a new technology, some XML parser do not support it yet.