A schema is a collection of database objects. A schema is owned by a database user and has the same name as that user. Schema objects are logical structures created by users to contain, or reference, their data. Schema objects include structures like tables, views, and indexes. You can create and manipulate schema objects using Oracle Enterprise Manager.

DTD stands for Document Type Definition and not Data Type Definition.
In addition to Document type definitions, Schema supports data type definitions too.
XML 1.0 does not provide option to use namespace in DTD's. whereas Schema provides