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How ontologies differ from other information models
RDFS and OWL are languages with clearly defined semantics or mathematical basis for the meaning of each construct. Semantics are defined in terms of inferences each statement entails. Since concepts in RDFS and OWL ontologies are expressed formally, they can be processed by computer programs. Because traditional modeling techniques (such as entity relationship) describe concepts only semi-formally, they cannot be handled automatically by software without significant human programming effort to make their meanings explicit.
Similarities and differences between ontologies and other types of information models can be described in the following way:
Like databases, ontologies are used by applications at run time (queried and reasoned over) --Unlike databases, relationships are first-class constructs --Unlike databases, ontologies can easily change their schema at run-time
Like object models, ontologies describe classes and attributes (properties) --Unlike object models, ontologies are set-based
Like business rules, ontologies encode rules --Unlike business rules, ontologies organize rules using axioms
Like XML schemas, ontologies are native to the web (and can be serialized in XML) --Unlike XML schemas, ontologies are graphs not trees and can be used for reasoning