If you've read the first couple of installments of this series, you should have a pretty good idea of what the "semantic web" is by now. By providing precise meaning to a site's content, applications can take advantage of these machine-readable hints to link data together across sites in a myraid of ways. Before you jump in the deep end of the semantic web pool, there's a few more things you should have a clear understanding of.
As I previously defined, RDF stands for Resource Description Framework. This is a family of standards for describing content on the web. The vast majority of current and future Drupal implementations of RDF are actually RDFa (Resource Description Framework in Attributes), a set of extensions to XHTML. RDF is normally implemented using XML; while this is possible with Drupal, RDFa allows Drupal to implement RDF as part of the standard content displays.