A family of international standards for data interchange on the web. RDF is based on the idea of identifying things using web identifiers or HTTP URIs and describing resources in terms of simple properties and property values.

Source: W3C — http://www.w3.org/TR/ld-glossary/#rdf

Resource Description Framework, the native way of describing linked data. RDF is not exactly a data format; rather, there are a few equivalent formats in which RDF can be expressed, including an XML-based format. RDF data takes the form of ‘triples’ (each atomic piece of data has three parts, namely a subject, predicate and object), and can be stored in a specialised database called a triple store. 

Source: ODH

A family of specifications for a metadata model. The RDF family of specifications is maintained by the World Wide Web Consortium (W3C). The RDF metadata model is based upon the idea of making statements about resources in the form of a subject-predicate-object expression…and is a major component in what is proposed by the W3C’s Semantic Web activity: an evolutionary stage of the World Wide Web in which automated software can store, exchange, and utilize metadata about the vast resources of the Web, in turn enabling users to deal with those resources with greater efficiency and certainty. RDF’s simple data model and ability to model disparate, abstract concepts has also led to its increasing use in knowledge management applications unrelated to Semantic Web activity.

Source: US OD