One of the most fundamental de facto standards of the Semantic Web is the Resource Description Framework (RDF), a powerful knowledge representation language. The structured data of ontologies, knowledge bases, and datasets, as well as the lightweight annotations of web sites are usually expressed in, or based on, one of the syntax notations and data serialization formats of RDF. RDF is suitable for creating machine-interpretable descriptions on any web resource by incorporating factual data and definitions from arbitrary external vocabularies, knowledge bases, ontologies, and datasets. The vocabulary or ontology to use depends on the area of interest you want to represent; however, some knowledge domains such as persons and movies can be described with classes and properties from more than one vocabulary. One can also create new ontologies for those knowledge domains that are not covered by existing ontologies.
The RDF standard provides a vocabulary and constructors for machine-interpretable statements, a graph representation for statements with IRIs, plain and typed literals, blank nodes (for anonymous resources), and several syntaxes.
The RDF data model is based on statements to describe and feature resources, especially web resources, in the form of subject–predicate–object (resource–property–value) expressions called RDF triples or RDF statements, as for example,
video.mp4 – title – “The Notebook” or
MovieClip – runningTime – 124.
The predicate annotates a relationship between the subject and the object, which addresses a limitation of HTML and XML.
The format and serialization of structured data are independent from the vocabulary definitions, so, for example, the same schema.org reference can be used in RDF/XML, Turtle, RDFa, HTML5 Microdata, JSON-LD, and so on.