Stands for "Resource Description Framework." RDF is a specification that defines how metadata, or descriptive information, should be formatted. The RDF model uses a subject-predicate-object format, which is a standardized way of describing something. For example, an RDF expression may read, "The computer has a hard drive that stores 250GB." "The computer" is the subject, "has a hard drive that stores" is the predicate, and "250GB" is the object.
RDF formatting is used in RSS feeds, which contain short descriptions of Web pages. The RDF standard helps ensure each description contains the subject, predicate, and object necessary to describe the page's content. While humans do not require descriptions to be formatted in such a specific way (we would actually find it rather monotonous), computers benefit from the standard formatting. For example, it makes it easier for computer systems to sort and index RSS feeds based on the RDF descriptions. The end result is more accurate results when people search for articles using keywords.