A Resource Description Foundation (RDF) vocabulary is a defined set of predicates that can be used in an application. You can define a vocabulary for your application by creating an ontology file, which is an RDF document that contains all possible predicates for an application. An ontology not only defines the predicates themselves, but defines the data type of each predicate and the relationship, if any, of one predicate to another.
A vocabulary is similar to a schema in that it defines a set of elements that can be used in an application. However, a vocabulary also helps you to define the domain and range of a predicate. You can define a domain to indicate that the predicate is of a particular RDF class. You can define a range to indicate that the predicate is of a particular data type. For example, if you have defined a predicate in an ontology file named UserInfo, you can create an e-mail-address predicate that is of type UserInfo in your ontology document. You could also define the data type, or range, of the E-mail-Address predicate.
- What is RDF Vocabulary? - MSDN Library