This document discusses key concepts in ontology including: - Ontology is the formal representation of objects, entities and their relationships in a domain. It includes a vocabulary and definitions of terms and how they are related. - There are reference ontologies which provide rich theories of a domain and application ontologies which provide minimal structures for a specific community. - Continuants have continuous existence while occurrents unfold over time. There are independent entities like organisms and dependent qualities that require independent entities. - Classes represent general concepts while instances are particular entities. Relations connect classes and can also relate classes to instances.