4.2 Introduction to Ontologies

	OWL ontologies are used to model domain knowledge. This encompasses significantly more than the simple structure of an RDF graph or the list of defined terms and hierarchical structure of an RDFS vocabulary. Ontologies are the core element of the Semantic Web.
	Semantic Web technologies are designed to make the World Wide Web more machine-understandable.
	Resources on the Web are inherently distributed, and as a result, the resource descriptions contained on the Semantic Web are also distributed.
	OWL supports distributed knowledge model because it is built on RDF, which allows you to declare and describe resources locally or refer to them remotely. To provide a foundation on which to make valid inferences in the distributed knowledge model of the Semantic Web, we must make two important assumptions: the open world assumption and the no unique names assumption.
	OWL ontologies are commonly stored as documents on the Web. Each document consists of an optional ontology header, annotations, class and property definitions (more formally referred to as axioms), facts about individuals, and data type definitions.
	Since OWL is based on the RDF model, there is no explicit distinction between the ontology and the data the ontology is used to describe.
	Ontologies optionally contain headers that define and describe the resource representing the ontology itself.
	Ontologies are composed of three fundamental semantic building blocks: classes, individuals, and properties.

4.2 Introduction to Ontologies

		A class is a set of resources.
		An individual is any resource that is a member of at least one class.
		A property is used to describe a resource.

	Finally, ontologies can contain data type definitions that describe ranges of values.
	Elements of Ontology covering annotations and ontology declarations. We then move into basic classification, class membership, and property-based description. Finally, we explore the more complex features of the language, including defining data types, using property restrictions, defining classes using enumerations and set operators, and working with equality in OWL.