In addition to its formal semantics, semantic data has a simple data structure that is effectively modeled using a directed graph. The metadata statements are represented as triples: nodes are used to represent two parts of the triple, and the third part is represented by a directed link that describes the relationship between the nodes.
Statements are expressed in triples:
{subject or resource, predicate or property, object or value}.
In this manual, {subject, property, object}
is used to describe a triple, and the terms statement and
triple may sometimes be used interchangeably. Each triple is a
complete and unique fact about a specific domain, and can be
represented by a link in a directed graph.
The RDF Graph feature supports the use of named graphs, which are described in the "RDF Dataset" section of the W3C SPARQL Query Language for RDF recommendation (http://www.w3.org/TR/rdf-sparql-query/#rdfDataset).
This support is provided by extending an RDF triple consisting of the traditional subject, predicate, and object so as to include an additional component representing a graph name. The extended RDF triple, despite having four components, will continue to be referred to as an RDF triple in this document. In addition, the following terms are sometimes used:
N-Triple is a format that does
not allow extended triples. Thus, n-triples can
include only triples with three components.
N-Quad is a format that allows both "regular" triples
(three components) and extended triples (four components,
including the graph name). For more information, see
N-Quads: Extending N-Triples with Context.
The graph name component of an RDF triple must either be null or a URI. If it is null, the RDF triple is said to belong to the default graph; otherwise it is said to belong to the named graph whose name is designated by the URI.