Ontologies

1. Knowledge Representation
 - Knowledge representation is central to the Semantic Web
 - Long-standing concern in Artificial Intelligence
 - Most AI systems (and therefore the Semantic Web systems) consist of ...
  1) a knowledge base (KB): structured according to the knowledge representation approach taken
  2) an inference mechanism: set of procedures that are used to examine the knowledge base to answer questions solve problems or make decisions within the domain

 https://en.wikipedia.org/wiki/Knowledge_representation_and_reasoning


2. Ontologies
 - An ontology is a specification of a conceptualisation
  1) specification: a formal description
  2) conceptualisation: the objects, concepts, and other entities that are assumed to exist in some area of interest and the relationships that hold among them

 - Ontology in Computer Science
  1) constituted by a specific vocabulary used to describe a certain reality
  2) a set of explicit assumptions regarding the intended meaning of the vocabulary
  3) benefits: shared understanding / facilitate communication / inter-operability

 - Ontology Structure: typically have two distinct components
  1) Names for important concepts in the domain
  2) Background knowledge/constraints on the domain


3. Informal Usage
 : Informally, 'ontology' may also be used to describe a number of other types of conceptual specification

 - Controlled Vocabularies
  1) An explicitly enumerated list of terms, each with an unambiguous, non-redundant definition
  2) no structure exists between terms - a flat list
  e.g. Library of Congress Subject Headings (LCSH)

 - Taxonomies
  1) a collection of controlled vocabulary terms organised into a hierarchical structure
  2) each term is in one or more parent-child relationships
  e.g. Library Classification schemes(Library of Congress, Dewey Decimal, UDC)

 - Thesauri
  1) a taxonomy with additional relations showing lateral connections: Related Terms (RT) and See Also
  2) parent-child relation usually described in terms of Broader Terms (BT) and Narrower Terms (NT)

 - Ontology
  1) an ontology further specialises types of relationships (particularly related terms)
  2) typically includes class definitions and hierarchy, relation definitions ad hierarchy
  3) may also include constraints, axioms and rule-based knowledge

 - Summary
  1) Controlled Vocabulary + Hierarchy = Taxonomy
  2) Taxonomy + later relations = Thesaurus
  3) Thesaurus + typed relations + constraints + rules + axioms = Ontology

http://www-ksl.stanford.edu/kst/what-is-an-ontology.html

http://protege.stanford.edu/publications/ontology_development/ontology101-noy-mcguinness.html