RDFS
RDFS is the schema vocabulary that sits on top of RDF: classes and subclasses, properties and their domain and range, with simple entailment rather than validation.
What it is
RDFS — the RDF Schema vocabulary — adds terms that let an RDF graph describe itself: rdfs:Class and rdf:type group things, rdfs:subClassOf places a class under another, and rdf:Property, rdfs:domain and rdfs:range describe predicates and what they connect. RDFS entailment says what follows from a graph that uses these terms.
What it gives the knowledge layer
It gives a graph a vocabulary for its own terms at low cost: a team can publish classes, a hierarchy and property definitions without adopting a reasoning-heavy ontology, and a processor can derive type memberships that were never stated.
Where the spec lives
The vocabulary is defined in RDF Schema 1.1, a W3C Recommendation listing the classes and properties and what each means. The entailment rules are part of RDF 1.1 Semantics: the schema document says what terms exist, the semantics document what a processor may derive.
What it does not solve
Domain and range are not constraints but inferences: saying that a property has a domain asserts that anything used as its subject is a member of that class, and does not reject a triple that looks wrong. A graph containing a mistake is used to derive further statements, so RDFS will not keep bad data out. Cardinality and disjointness are what OWL addresses.
FAQ
No. RDFS produces inferences, not error reports. It will accept a triple that looks wrong to a human and use it to derive further statements.
It states that every instance of one class is an instance of another, letting a processor derive the second membership from the first.
Inferences. Saying that a property has a domain asserts that anything used as its subject is a member of the class named; data that breaks the schema makes the processor conclude something extra.
When the need is naming and a light hierarchy: terms, classes, parent-child relations and enough typing for a query to traverse. For cardinality or disjointness, the work moves up to OWL.
Sources
- RDF Schema 1.1 — W3C Recommendation defining the RDFS classes and properties.
- RDF 1.1 Semantics — W3C Recommendation defining RDFS entailment and the other interpretation regimes for RDF graphs.