"Ontologies provide a shared and common understanding of a domain that can be communicated between people and application systems," writes the author. A schema, meanwhile, "provides [the] basic vocabulary and predefined structuring mechanisms for providing information in XML." Consequently, "Ontologies applied to on-line information source may be seen as explicit conceptualizations that describe the semantics of the data." In other words, a schema defines what properties an object can have, while an ontology specifies the possibile values of those properties. So (very roughly) a schema would say that a car could have a "colour" while an ontology wuld say that a "colour" could be "red, orange, yellow, green, blue or purple." Ontologies can be defined using a language called OIL, or Ontology Interface Layer. The bulk of the article is devoted to explaining OIL, comparing it with XML Schemas, and showing how the two language systems interact. Tough, technical, but well worth reading.
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