Through RDF, independent communities can develop vocabularies that suit their specific needs, and share vocabularies with other communities. In order to share vocabularies, term meanings must be spelled out in detail. The descriptions of these vocabulary sets are call XML Schemas. XML Schemas XML Schemas may eventually supplant DTDs as the primary mechanism for constraining XML data. An XML Schema, which is the format of an XML document, serves the same function as a DTD while correcting some of its limitations.
Schemas give developers more powerful data typing for both element content and attribute values.
XML Schemas supplement the basic DTD mechanism included in XML Version 1.0 with a more rigorous framework for declaring structure and contents of XML documents. Since XML 1.0 doesn't support the inclusion of semantic information about the format of character data, the XML parser wouldn't know that the value Green was supposed to be a date field. The date range could also be constrained. A schema defines the meaning, characteristics, and relationships of a set of properties. The RDF language allows each document containing metadata to clarify which vocabulary is being used by assigning each vocabulary a Web address.