Ontology is a study of conceptions of reality and the nature of being. In philosophy, ontology (from the Greek ὤν, genitive ὄντος: of being (part. of εἶναι: to be) and -λογία: science, study, theory) is the study of being or existence and forms the basic subject matter of metaphysics. It seeks to describe or posit the basic categories and relationships of being or existence to define entities and types of entities within its framework.
Its sister study, epistemology is a study of knowledge, including the search for answers to questions concerning how knowledge is acquired and maintained.
As a philosophical subject, ontology chiefly deals with the precise utilization of words as descriptors of entities or realities. 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, happiness, time, truth, causality, and God, ontology becomes fundamental to many branches of philosophy.

Some basic questions
Quintessential ontological concepts include:

Universals
Substance Concepts
The concept of ontology is generally thought to have originated in early Greece. Before Socrates, questions of being, stasis and change occupied Parmenides and Heraclitus. Parmenides is associated with the view that being must be affirmed and non-being avoided and denied. This is also expressed by Parmenides in the dictum "ἔστι γὰρ εἶναι" or "It is, namely, being." Parmenides denied that there is any real change in the universe, and Heraclitus is diammetrically opposed to Parmenides in his affirmation of change as the ultimate nature of things.
After Socrates, ontology was very important for Plato and Aristotle. While the etymology is Greek, the oldest extant record of the word itself is the Latin form ontologia, which appeared in 1661, in the work Ogdoas Scholastica by Jacob Lorhard (Lorhardus) and in 1631 in the Lexicon philosophicum by Rudolph Göckel (Goclenius). The first occurrence in English of "ontology" as recorded by the OED appears in Bailey's dictionary of 1721, which defines ontology as 'an Account of being in the Abstract'. However its appearance in a dictionary indicates it was in use already at that time. It is likely the word was first used in its Latin form by philosophers based on the Latin roots, which themselves are based on the Greek.
Students of Aristotle first used the word 'metaphysica' (literally "after the physics" because these works were placed after his works on physics) to refer to the work their teacher described as "the science of being qua being". The word 'qua' means 'in the capacity of'. According to this theory, then, ontology is the science of being inasmuch as it is being, or the study of beings insofar as they exist. 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 precisely, 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".
Ontological questions have also been raised and debated by thinkers in the ancient civilizations of India and China, in some cases perhaps predating the Greek thinkers who have become associated with the concept.

Early history of ontology
"What exists", "What is", "What am I", "What is describing this to me", all exemplify questions about being, and highlight the most basic problems in ontology: finding a subject, a relationship, and an object to talk about. During the Enlightenment the view of René Descartes that "cogito ergo sum" ("I think therefore I am") had generally prevailed, although Descartes himself did not believe the question worthy of any deep investigation. However, Descartes was very religious in his philosophy, and indeed argued that "cogito ergo sum" proved the existence of God. Later theorists would note the existence of the "Cartesian Other" — asking "who is reading that sentence about thinking and being?" — and generally concluded that it must be God.
This answer, however, became increasingly unsatisfactory in the 20th century as the philosophy of mathematics and the philosophy of science and even particle physics explored some of the most fundamental barriers to knowledge about being. Sociological theorists, most notably George Herbert Mead and Erving Goffman, saw the Cartesian Other as a "Generalized Other," the imaginary audience that individuals use when thinking about the self. The Cartesian Other was also used by Freud, who saw the superego as an abstract regulatory force.

Subject, relationship, object
Schools of subjectivism, objectivism and relativism existed at various times in the 20th century, and the postmodernists and body philosophers tried to reframe all these questions in terms of bodies taking some specific action in an environment. This relied to a great degree on insights derived from scientific research into animals taking instinctive action in natural and artificial settings — as studied by biology, ecology, and cognitive science.
The processes by which bodies related to environments became of great concern, and the idea of being itself became difficult to really define. What did people mean when they said "A is B", "A must be B", "A was B"...? Some linguists advocated dropping the verb "to be" from the English language, leaving "E Prime", supposedly less prone to bad abstractions. Others, primarily philosophers, tried to dig into the word and its usage. Heidegger attempted to distinguish being and existence.

Body and environment
Issues of an ontological nature become of concern regarding discussions about possible descriptions of God. Anselm's description of God is of "that which nothing greater can be conceived". Anselm argues the 'a priori' argument for the existence of God, stating that if God is "that which nothing greater can be conceived" of, he would have to exist for this description to be correct. Something which exists is greater than that which is imagined. God would necessarily, according to Anselm, exist, as we could possibly imagine something to be greater than God, which would therefore be impossible. Anselm's argues that as things which exist are greater than those which are imagined, God would have to, therefore exist for his description to be accurate. The counter-argument states that, just because one cannot think of something greater than God does not mean that there is nothing greater, except by arbitrary definition: "God = That which nothing greater can be conceived." This is a circular argument, using a definition that limits the possible answers (thus limiting God, which is then problematical); the cognitive powers of mind transcend actual reality all the time (contrary to the concept that something real is greater than something imagined). One might also take the stance that God is something that is beyond Conception; World Religions in common practice define their God(s) as: without beginning or end, or Not Born, and therefore, Not Mortal.

Anselm's Ontological Argument for the existence of God
Many forms of existentialism regard being as a fundamental central concept. Heidegger had much to say on the matter of being. The verb to be has many different meanings in different contexts and can therefore be rather ambiguous. Because "to be" has so many different meanings, there are, accordingly, many different ways of being..
Descartes argues that God is a 'supremely perfect being' and that existence is a perfection so therefore God must exist. This also links to Descartes cogito ergo sum 'I think therefore I am' stating that we are thinking things and therefore exist in some unarguable form.

Being and non-being
The first formal development of this notion within philosophy began with the pre-Socratic Heraclitus, where he posited agon ("strife of opposites") as the ontological basis of all reality in terms of this endless transformative conflict, which was later contrasted and dominated by the Parmenidean, or Platonic, notion of Being, until more recent philosophers began a reversion of this trend.
Notably and the first to make such an advocation since Heraclitus was the nineteenth century German philosopher Friedrich Nietzsche, who used the expression "the innocence of becoming", a fundamental element of his philosophical thought grounded in the "will to power as pathos", as a means to describe the aesthetic qualities of existence, which pervades his thinking, including but not limited to ideas such as his "Dionysian world", "eternal recurrence", "amor fati", and "decadence". It was with this a-teleological view that he attempted to disgregate all views pertaining to the human condition, where "thingness" is ultimately characterized as a mere "hypothesis" in Nietzsche's phrase, and such a view, pertaining to the "inequality" of all "things", carries deep implications for ethics and the nature of knowledge.
Likewise, in his Logic, Hegel uses Becoming as a mediating force in his dialectical model of ontology. In this model, Being is, on the one hand, opposing to Non-Being and, on the other hand, "is the same as Non-Being." Becoming acts therefore as the process by which Being comes into itself, or "becoming is the unity of being and not-being." [1]

Becoming

Main article: Process philosophy Process philosophy
Social scientists adopt one of four main ontological approaches: realism (the idea that facts are out there just waiting to be discovered), empiricism (the idea that we can observe the world and evaluate those observations in relation to facts), positivism (which focuses on the observations themselves, attentive more to claims about facts than to facts themselves), and postmodernism (which holds that facts are fluid and elusive, so that we should focus only on our observational claims).