RDF, or the Resource Description Framework one of of the tools currently being used to explor and build the SemanticWeb. Interesting applications that use it are Chandler (from the Open Source Applications Foundation) and IdeaGraph. I'm sure there are many more.
Their is a W3C has the official docs and a primer available:
Books
O'Reilly has a book called "Practical RDF" (AmazonIsbn:0596002637) by ShellyPowers (aka. Burning Bird). The home page for the book is here:
Other Resources
Introductions:
RDF Interest Group Resources:
- IRC on irc.freenode.net, #rdfig
JoWalsh's web site:
Semaview's RDF and XML slide
RDF and XML (jpg)
ShellyPower's thoughts:
LesOrchard's Notes: