What are the core technologies behind the Semantic Web?

The core technologies behind the Semantic Web are RDF, OWL, SPARQL, and XML.

The Resource Description Framework (RDF) is a fundamental technology of the Semantic Web. It's a standard model for data interchange on the Web. RDF extends the linking structure of the Web to use URIs to name the relationship between things as well as the two ends of the link. This linking structure forms a directed, labelled graph, where the edges represent the named link between two resources, represented by the graph nodes. This graph view is the easiest possible mental model for RDF and is often used in easy-to-understand visual explanations.

The Web Ontology Language (OWL) is another key technology. OWL is a Semantic Web language designed to represent rich and complex knowledge about things, groups of things, and relations between things. OWL is a computational logic-based language, which means that the concepts in OWL can be interpreted computically. The language is built on RDF and adds more vocabulary for describing properties and classes, such as relations between classes, cardinality, equality, richer typing of properties, characteristics of properties, and enumerated classes.

SPARQL (pronounced "sparkle") is a query language for RDF. It provides the ability to query data across various systems, whether the data is stored natively as RDF or viewed as RDF via middleware. SPARQL contains capabilities for querying required and optional graph patterns along with their conjunctions and disjunctions. It also supports aggregation, subqueries, negation, creating values by expressions, extensible value testing, and constraining queries by source RDF graph.

Lastly, the Extensible Markup Language (XML) is a markup language that defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. It is a textual data format with strong support via Unicode for different human languages. Although the design of XML focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services. Many application programming interfaces (APIs) have been developed to aid software developers with processing XML data.

These technologies work together to create the Semantic Web, a web of data that can be processed directly and indirectly by machines.