 As a first approximation, ontology is the study of what there is. Some contest this formulation of what ontology is, so it’s only a first approximation. Many classical philosophical problems are problems in ontology: the question whether or not there is a god, or the problem of the existence of universals, etc.. These are all problems in ontology in the sense that they deal with whether or not a certain thing, or more broadly entity, exists. But ontology is usually also taken to encompass problems about the most general features and relations of the entities which do exist. There are also a number of classic philosophical problems that are problems in ontology understood in this way. For example, the problem of how a universal relates to a particular that has it (assuming there are universals and particulars), or the problem of how an event like John eating a cookie relates to the particulars John and the cookie, and the relation of eating, assuming there are events, particulars and relations. These kinds of problems quickly turn into metaphysics more generally, which is the philosophical discipline that encompasses ontology as one of its parts. The borders here are a little fuzzy. But we have at least two parts to the overall philosophical project of ontology, on our preliminary understanding of it: first, say what there is, what exists, what the stuff of reality is made out of, secondly, say what the most general features and relations of these things are.