An ontology is a specification of a conceptualization.
Theories in all AI fall into TWO broad categories: mechanism theories and content theories. Ontologies are content theories about the sorts of objects, properties of objects, and relations between objects that are possible in a specified domain of knowledge. They provide potential terms for describing our knowledge about the domain. Most research on ontologies focuses on what one might characterize as domain factual knowledge, because knowlede of that type is particularly useful in natural-language understanding. There is another class of ontologies that are important in KBS—one that helps in sharing knoweldge about reasoning strategies or problemsolving methods.
Ontology: The branch of metaphysics (philosophy concerning the overall nature of what things are) is concerned with identifying, in the most general terms, the kinds of things that actually exist. In other words addressing the question: What is existence? and What is the nature of existence? When we ask deep questions about "what is the nature of the universe?" or "Is there a god?" or "What happens to us when we die?" or "What principles govern the properties of matter?" we are asking inherently ontological questions.
Epistemology: The branch of philosophy concerned with the nature of knowledge itself, its possibility, scope, and general basis. More broadly: How do we go about knowing things? or How do we separate true ideas from false ideas? or How do we know what is true? or "How can we be confident when we have located 'truth'?" "What are the systematic ways we can determine when something is good or bad?"
So ontology is about what is true and epistemology then is about methods of figuring out those truths.
Protego:
https://wiki.csc.calpoly.edu/OntologyTutorial/wiki/IntroductionToOntologiesWithProtege
Keywords:
OWL
RDF