We need Models to describe the World (or just a single Domain). This can be done with the [Resource Description Framework](/blog/514/) Schema (RDFs) or the [Web Ontology Language](/blog/764/) (OWL).
In RDFs it is defined/stated what a class really is. The Models are build up with tiny little Sentences, i.e. Facts, consisting out of Subject, [Property](/blog/622/) and [Object](/blog/562/). These Facts can also be represented in a Graph Representation.
