Ontology and Epistemology
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.
The split between Plato and Aristotle is both ontological and epistemic. The split between religion and science is both ontological and epistemic. For example, religion and science offer two very different ontologies (theories about what is out there) and epistemology (ways to figure out what is out there). And the split between Plato and Aristotle matches exactly the split between religion and science...and you should leave this class understanding why and how! See Plato vs. Aristotle