RDF is a graph data model created by W3C, intended to provide a universal standard for information available on the web and make different sources easier to merge with each other. It is the basis for concepts like SPARQL and OWL, and the foundation of the Semantic Web.
An RDF database is comprised of subject-predicate-object triples that form a simple directed graph. Each element of a triple is either a literal, a “blank node” or a URI (uniform resource identifier). Since URIs are a superset of URLs, this is meant to help the user locate web pages corresponding to resources in the graph.
Though there is no notion of a schema, the contents of an RDF database can be described through an ontology.