RDF is a simple Model, in the sense that is always a
set of RDF Statements. The Statements in RDF always consists of three parts, a Subject, Property and an Object. A Subject is always a resource identified by an URI, a Property is also a resource identified by an URI, while the Object might be either a Resource or a Literal. By combining that one can make really extensible and expressive knowledge bases.
With RDS alone one can only state knowledge about facts, but only about simple facts. However, one can not yet refer to or build a model that serves somehow as a Schema or a Layout for Facts. By defining Classes and Relationships among Classes one cannot do it by simple RDF, one needs some kind of Schema Language. So we need a way to define Schemata for Classes and Relations.