I've been reading about Semantic Web technologies such as RDF/RDFS and "ontology", but was wondering how each of these are related? At the moment the terms all seem interchangeable, or I'm not understanding a fundamental concept here.
RDF is a data model, in a similar way to how JSON is a data model.
Notably RDF has a number of save formats (serialization formats), including RDF/XML, N3, Turtle, and others.
RDFS is a vocabulary of widely used terms (URIs) to define classes and relationships, etc.
Ontologies are generally written using RDFS terms or perhaps terms from something more powerful like OWL or OWL2.