RDF

RDF seems like as good a place to start as any. RDF is explained in terms of subjects, objects and predicates. Predicates link objects to subjects where the subject is the thing, the object is the value and the predicate is the property. However objects can also be subjects so to me it seems easier to think of RDF consisting of two items, objects and predicates where predicates link objects to each other. Forget this notion of subjects altogether.

An RDF graph then is the structure created when one tries to visualize a set of connected objects. There are a number of different ways to represent this graph, one is to draw it, another it to use what are called tuples (just specially formatted pieces of text) and finally one can use XML. RDF/XML is the standard way of representing an RDF graph in XML.

We that's all I'm going to say for now. On the W3C website there are a series of 6 articles describing RDF. For anyone interested in RDF but without much knowledge already I recommend you read at least section 1 and 2 of the Primer.

Of course you could just google this but the six documents in the series are:
Primer
Concepts
Syntax
Semantics
Vocabulary
Test Cases

The wikipedia entry is also worth a read although it makes a lot more sense after the Primer.