The term "Semantic Web" is often used more specifically to refer to the formats and technologies that enable it. The collection, structuring and recovery of linked data are enabled by technologies that provide a formal description of concepts, terms, and relationships within a given knowledge domain. These technologies are specified as W3C standards and include:
· Resource Description Framework (RDF), a general method for describing information
· RDF Schema (RDFS)
· Simple Knowledge Organization System (SKOS)
· SPARQL, an RDF query language
· Notation3 (N3), designed with human-readability in mind
· N-Triples, a format for storing and transmitting data
· Turtle (Terse RDF Triple Language)
· Web Ontology Language (OWL), a family of knowledge representation languages
The Semantic Web Stack illustrates the architecture of the Semantic Web. The functions and relationships of the components can be summarized as follows:
· XML provides an elemental syntax for content structure within documents, yet associates no semantics with the meaning of the content contained within. XML is not at present a necessary component of Semantic Web technologies in most cases, as alternative syntaxes exists, such as Turtle. Turtle is a de facto standard, but has not been through a formal standardization process.
· XML Schema is a language for providing and restricting the structure and content of elements contained within XML documents.
· RDF is a simple language for expressing data models, which refer to objects ("resources") and their relationships. An RDF-based model can be represented in a variety of syntaxes, e.g., RDF/XML, N3, Turtle, and RDFa. RDF is a fundamental standard of the Semantic Web.
· RDF Schema extends RDF and is a vocabulary for describing properties and classes of RDF-based resources, with semantics for generalized-hierarchies of such properties and classes.
· OWL adds more vocabulary for describing properties and classes: among others, relations between classes (e.g. disjointness), cardinality (e.g. "exactly one"), equality, richer typing of properties, characteristics of properties (e.g. symmetry), and enumerated classes.
· SPARQL is a protocol and query language for semantic web data sources.
