What is 'RDF'?
RDF stands for Resource Description Framework and is a language for describing resources on the World-Wide Web. It has been designed to represent metadata about Web resources, such as the title, author, and modification date of a Web page. It could equally describe a web document's copyright and licensing information, or the availability schedule for some shared resource.
Although RDF was originally conceived for the purpose of describing resources that are available on the web, there is nothing to stop you from using it as a language to describe other resources that are not available on-line. Therefore RDF is often seen as a lightweight language for describing ontologies.
You can find out more on RDF by reading the in-depth W3C primer, or by reading the much shorter article What is RDF?.