Ontology

Ontology in philosophy (from the Greek science, study, theory) is the study of the nature of being, existence or reality in general, as well as of the basic categories of being and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics, ontology deals with questions concerning what entities exist or can be said to exist, and how such entities can be grouped, related within a hierarchy, and subdivided according to similarities and differences.
Students of Aristotle first used the word 'metaphysica' (literally "after the physical") to refer to what their teacher described as "the science of being qua being" - later known as ontology. 'Qua' means 'in the capacity of': hence, ontology is inquiry into being in so much as it is being, or into being in general, beyond any particular thing which is or exists; and the study of beings insofar as they exist, and not insofar as, for instance, particular facts obtained about them or particular properties relating to them.
Take anything you can find in the world, and look at it, not as a puppy or a slice of pizza or a folding chair or a president, but just as something that is. More specifically, ontology concerns determining what categories of being are fundamental and asks whether, and in what sense, the items in those categories can be said to "be".Some philosophers, notably of the Platonic school, contend that all nouns refer to existent entities. Other philosophers contend that nouns do not always name entities, but that some provide a kind of shorthand for reference to a collection of either objects or events.
In this latter view, mind, instead of referring to an entity, refers to a collection of mental events experienced by a person; society refers to a collection of persons with some shared characteristics, and geometry refers to a collection of a specific kind of intellectual activity. Between these poles of realism and nominalism, there are also a variety of other positions; but any ontology must give an account of which words refer to entities, which do not, why, and what categories result. When one applies this process to nouns such as electrons, energy, contract, happines, space, time, truth, causality, and god, ontology becomes fundamental to many branches of philosophy.