What is the difference between RDF Schema and Ontology?

Question

I am new to Semantic Web and confused regarding RDFs and Ontology. Can someone explain the difference between RDF Schema and Ontology?

Answer 1

As per the spec, RDF schema is purely that - a schema or structure for defining things semantically. It gives you the vocabulary (key words and properties) for describing things. Think of it like an XML schema as used in XML documents and web pages.

An ontology is a classification hierarchy (for example, the biological taxonomy of life) normally combined with instances of those classes. It is used for classifying and reasoning.

What is an instance depends on how you define a taxonomy. It might be that you have an ontology of living creatures and so a living, breathing person is an instance of the ontological class "Homo Sapiens", or it might be that you have an ontology of species and so the entire Homo Sapiens species is an instance of the ontological class "Species".

Answer 2

Conceptually there is no difference, i.e., RDFS can be utilised to create a (e.g. domain specific) vocabulary or ontology, where RDFS is bootstrapping itself in companion with RDF (everything is at least an rdfs:Resource). Furthermore, in the context of Semantic Web technologies you could utilise OWL to describe advanced semantics of your ontology/vocabulary. See also this definition of ontology.

Answer 3

Do you mean 'what is the difference between RDF Schema' and 'Web Ontology Language (OWL2)'. If so then there are a few main differences. Both are ways to create vocabularies of terms to describe data when represented as RDF. OWL2 and its subsets (OWL DL, OWL Full, OWL Lite) contain all the terms contained in RDFS but allow for greater expressiveness, including quite sophisticated class and property expressions. In additional, one of the subsets of OWL2 (OWL Full) can be modelled in such a way that when reasoned using an OWL Full reasoner, is undecidable. Both are representable as RDF and both are W3C Web Standards.

If you want to compare RDFS and ontology, not specifically in the context above, but in the context of Semantic Web, then my advice would be to very careful. Careful because you will find several distinct and not necessarily mutually exclusive camps; those with an interest in ontology from a philosophical perspective, those from a computing perspective, those who think the philosophical perspective should be the only perspective and those that don't. If you are any of those ways inclined, you can end up having great debates. But if you want to engage in Semantic Web Development, then the fastest route is to study and understand the Web Standards mentioned initially.

Answer 4

RDF Schema (RDFS) is a language for writing ontologies.

An ontology is a model of (a relevant part of) the world, listing the types of object, the relationships that connect them, and constraints on the ways that objects and relationships can be combined.

A simple example of an ontology (though not written in RDFS syntax):

class: Person
class: Project
property: worksOn

worksOn domain Person
worksOn range Project

which says that in our model of the world, we only care about People and Projects. People can work on Projects, but not the other way around.

Answer 5

In non technical terms, I would say RDFS is a language that helps to represent information. And an ontology is the term used to refer to all the information about a domain.

Cheers