An ontology consists of a set of concepts and the relation-
ships between them [11]. It is a formal representation of the
common vocabulary of a knowledge area. These concepts
and their relationships are under an agreement within an
area of knowledge or across different areas of knowledge.
A concept is exemplied by specic instances. For exam-
ple, the admission fees of an attraction are $25. Here the
`admission fees' is a concept, `the admission fees are $25' is
an instance of the concept. Values associated with concepts,
such as `$25' associated with `admission fees', are referred
to as properties of concepts in this discussion. Based on
ontologies, concepts that represent different perspectives can
be compared and mapped through the similarities between
them.