XML file

. An XML file contains data and metadata. An XML file can reference a Document Type Definition file (DTD) or an XML schema definition (XSD) for validation.

DTD file

. A DTD file defines the element types, attributes, and entities in an XML file. A DTD file provides some constraints on the XML file structure but a DTD file does not contain any data.

XML schema.

An XML schema defines elements, attributes, and type definitions. Schemas contain simple and complex types. A simple type is an XML element or attribute that contains text. A complex type is an XML element that contains other elements and attributes.

Schemas support element, attribute, and substitution groups that you can reference throughout a schema. Use substitution groups to substitute one element with another in an XML instance document. Schemas also support inheritance for elements, complex types, and element and attribute groups.