Metadata is data that describes data. One widely known example--and a precursor to the RDF--is the Platform for Internet Content Selection (PICS), which provides information about the type of content found on a given page. In the case of PICS, the metadata facilitates the ratings of Web sites according to age-appropriateness and other factors.
RDF aims to extend comparable information labeling to other kinds of Web-based information. As an example, any human reader can look at this page and distinguish between the author, the headline, the text of the article, and so forth. RDF enables computers to make those distinctions as well.
The most significant application of RDF for the every-day user is probably the search engine. Search engines canvassing RDF-fortified Web pages would do a much better job of sorting and presenting results to Web searchers, who today commonly come up with many thousands of irrelevant pages in their results.
Other applications of RDF include the cataloging of content, spelling out intellectual property rights of a Web page, and enhancing digital signatures, which verify users' identities.
RDF is based in part on Extensible Markup Language (XML), a metalanguage for the making of industry- or topic-specific markup languages. XML tags also provide data about data. But RDF is more flexible than XML in the way in which it organizes data descriptions, and therefore is a more successful framework for metadata.