I made a small RDF graph that looks as follows:
@prefix ex: http://example.org/ .
@prefix rdfs: http://www.w3.org/2000/01/rdf-schema# .
ex:a a ex:C, ex:S .
ex:C rdfs:subClassOf ex:D .
ex:D rdfs:subClassOf ex:E .
Now I am confused if I could call my RDF graph a knowledge graph as well or an Ontology. I tried to read various sources and those did not help me clear up my confusion.
The current scientific definition of the term 'Knowledge Graph' is from the Hogan et al. paper and says:
[...] a graph of data intended to accumulate and convey knowledge of the real world, whose nodes represent entities of interest and whose edges represent potentially different relations between these entities.
This is a quite inclusive definition and includes RDF Graphs. The linked paper talks about that in Section 2.
Your example is therefore a RDF Graph and a Knowledge Graph.
Regarding the term 'Ontology', the two subClassOf statements
ex:C rdfs:subClassOf ex:D .
ex:D rdfs:subClassOf ex:E .
would be part of an ontology (T-Box) that describe a schema for data.
The statements
ex:a a ex:C, ex:S .
would be instance data (A-box) using the ontology to describe the individual 'ex:a'.
It is fine to include both the ontology and instance data in one graph. However, I would be hesitant to call the short example you posted a fleshed out ontology as there are for example no properties and no metadata about the ontology.