Introducing RDF
A Resource is anything that can have a URI; this includes all the world's Web pages, as well as individual elements of an XML document
A PropertyType is a Resource that has a name and can be used as a property, for example Author or Title. In many cases, all we really care about is the name; but a PropertyType needs to be a resource so that it can have its own properties.
A Property is the combination of a Resource, a PropertyType, and a value