RDF and the Semantic Web " RDF is a framework for defining metadata; data that describes data. It was developed by the W3C, based on work by Ramanathan V. Guha, and was originally used in Netscape Navigator 4.5's Smart Browsing ("What's related?") feature, and by Open Directory. RDF followed from work Guha had done earlier, both on the Cyc project, and on Apple's Hotsauce project."
A very nice article from the pre-history of RDF to Web-of-trust, Co-depiction and DOAP.