RDF, RDF Schemas and Ontologies

The W3C's Resource Description Framework is one of the cornerstones of Semantic Web work.

the real value of RDF is the data model.

It defines a very simple data model of triples (subject, predicate, object), where subject and predicate are URIs, and the object is either a URI or a literal. With this simple model, objects and their properties may be represented.

RDF schemas differ somewhat from XML schemas (such as DTDs or W3C XML Schemas) in that they do not define a permissible syntax but instead classes, properties, and their interrelation: they operate directly at the data model level, rather than the syntax level. Scaled up over the Web, RDF schemas are a key technology, as they allow machines to make inferences about the data collected from the web.