•An ontology defines the terms used to describe and
 represent an area of knowledge.
 •Ontologies are used by people, databases, and
 applications that need to share domain
 information (a domain is just a specific subject area or area of knowledge, like medicine, tool manufacturing,
 real estate, automobile repair,
 financial management, etc.). 
 •Ontologies include computer-usable definitions of basic
 concepts in the domain and the relationships
 among them … They encode knowledge in
 a domain and also knowledge that spans domains. In this way, they make that knowledge reusable.
 •The word ontology has been used to describe artifacts
 with different degrees of structure.
 These range from simple taxonomies (such as the Yahoo hierarchy), to metadata schemes (such as the
 Dublin Core), to logical theories. The
 Semantic Web needs ontologies with a significant degree of structure.