"Typically a value vocabulary define concepts related to described resources (persons, languages, countries, etc.). They are "building blocks" with which metadata records can be populated.
A value vocabulary thus represents a controlled list of allowed values for an element. To be useful for linking of data, value vocabularies should have HTTP URIs assigned for each value; these URIs would then appear in a metadata record instead of or in addition to the literal value." W3C Library Linked Data Incubator Group

"Ontologies are formalized vocabularies of terms, often covering a specific domain and shared by a community of users. They specify the definitions of terms by describing their relationships with other terms in the ontology."
OWL 2 Web Ontology Language Document Overview (Second Edition) - W3C Recommendation 11 December 2012
"OWL Web Ontology Language is designed for use by applications that need to process the content of information instead of just presenting information to humans. OWL facilitates greater machine interpretability of Web content"
OWL Web Ontology Language Overview - W3C Recommendation 10 February 2004