Since the focus of this book is more on the practical usage of RDF than the more theoretical Semantic Web, I wasn't sure about covering ontologies. After all, in a white paper at Stanford University, Tom Gruber described ontology thus:
An ontology is a specification of a conceptualization.
It's a bit difficult to determine how to incorporate a discussion of a concept based on such an elusive definition into a book that begins with Practical. However, looking at examples of ontologies, in particular OIL, DAML+OIL, and the W3C's current OWL (Web Ontology Language) effort, it seemed to me that ontologies do fit into a book with Practical in the title, because an ontology is really the definition of the business rules associated with a vocabulary. In other words: ontologies are business models. According to the Web Ontology Language (OWL) Use Cases and Requirements document:
An ontology formally defines a common set of terms that are used to describe and represent a domain. Ontologies can be used by automated tools to power advanced services such as more accurate Web search, intelligent software agents and knowledge management
Following on the relational model analogy discussed in earlier chapters, if RDF is analogous to the relational data model and SQL is analogous to RDF/XML, then ontologies built on RDF/XML are equivalent to large architected business applications such as SAP, PeopleSoft, and Oracle's Financial and Warehouse applications. This equation definitely opened a home for ontologies in this book, and this chapter is it.