What is ontology? How is it compared with ER model and object-oriented model?
An ontology is a model of entity and relationship in a specific domain or universe of discourse (UoD). An ontology can vary from a taxonomy (knowledge with minimal hierarchy or a parent/child structure) to a thesaurus (words and synonyms) to a conceptual model (with more complex knowledge) to a logical theory (with very rich, complex, consistent and meaningful Knowledge). The current focus of the industry is on conceptual model and logical theory (such as OWL). Ontology is distinguishable from other models (such as UML and ER): It has formal semantics; that is, it has a model theory for its language. It supports inference, especially most ontology languages; and it is decidable, and even tractable.
