What is RDF?
The Resource Description Framework, or RDF, is an extremely flexible data model with numerous serializations and a powerful query language. RDF consists of ‘graphs’ and ‘triples’. Each triple comprises a ‘subject’, a ‘predicate’, and an ‘object’. Each graph contains a set of triples.
In RDF, every element is either a universally unique URI, a data literal, or ‘blank node’, whose identity is not known. This can make it seem verbose, but it’s critically important for machines when reading data from multiple sources.
A good summary is at http://www.rdfabout.com/quickintro.xpd, and a good comparison of modeling a simple piece of knowledge in a wide variety of data models is at http://www.w3.org/2000/10/swap/doc/formats.
The use of URIs allows multiple data sources to talk about the same entities using the same language. It’s a critical piece of the puzzle for making wildly different data sets talk to each other. None other than Tim-Berners Lee, who invented HTTP and thus the web, has written a good, short piece on why RDF’s interoperability characteristics are important.
RDF is a W3C standard, the original version of which was completed in 2004. The W3C maintains a list of relevant recommendations.
The original set of W3C standards blessed a particular serialization, RDF/XML, which we believe was a problem for RDF uptake. RDF is not a subset of XML, nor does it need to be as complicated as RDF/XML can be.
RDF is a data model, akin to the relational model we know from databases. Like relational databases you may be familiar with, the model has numerous competing implementations, built around a mostly-compatible query language. For RDF, this query language is SPARQL.