When discussing the Resource Description Framework (RDF) specification, we're really talking about two different specifications?a Syntax Specification and a Schema Specification. As described in Chapter 3 and Chapter 4, the Syntax Specification shows how RDF constructs relate to each other and how they can be diagrammed in XML. For instance, elements such as rdf:type and pstcn:bio are used to describe a specific resource, providing information such as the resource's type and the author of the resource. The different namespace prefixes associated with each element (such as rdf: and pstcn:) represent the schema that particular element is defined within.
In the context of RDF/XML, a vocabulary or schema is a rules-based dictionary that defines the elements of importance to a domain and then describes how these elements relate to one another. It provides a type system that can then be used by domain owners to create RDF/XML vocabularies for their particular domains. For example, the pstcn:bio element is from a custom vocabulary created for use with this book while the rdf:type element is from the RDF vocabulary. These are different vocabularies and have different vocabulary owners, but both follow rules defined within the RDF Vocabulary Description Language 1.0: RDF Schema.
However, before getting into the details of the RDF Schema, consider the following: if RDF is a way of describing data, then the RDF Schema can be considered a domain-neutral way of describing the metadata that can then be used to describe the data for a domain-specific vocabulary.
If all this seems convoluted, then you'll appreciate reading more about the concept of metadata, its importance to existing applications, and how RDF fits into the concept, all discussed in the next section.