Ontology is the philosophical

Philosophy

Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

 study of the nature of being

Being

Being , is an English word used for conceptualizing subjective and objective aspects of reality, including those fundamental to the self —related to and somewhat interchangeable with terms like "existence" and "living".In its objective usage —as in "a being," or "[a] human being" —it...

, existence

Existence

In common usage, existence is the world we are aware of through our senses, and that persists independently without them. In academic philosophy the word has a more specialized meaning, being contrasted with essence, which specifies different forms of existence as well as different identity...

or reality

Reality

In philosophy, reality is the state of things as they actually exist, rather than as they may appear or might be imagined. In a wider definition, reality includes everything that is and has been, whether or not it is observable or comprehensible...

as such, as well as the basic categories of being

Category of being

In metaphysics , the different kinds or ways of being are called categories of being or simply categories. To investigate the categories of being is to determine the most fundamental and the broadest classes of entities...

 and their relations. Traditionally listed as a part of the major branch of philosophy known as metaphysics

Metaphysics

Metaphysics is a branch of philosophy concerned with explaining the fundamental nature of being and the world, although the term is not easily defined. Traditionally, metaphysics attempts to answer two basic questions in the broadest possible terms:...

, 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

Hierarchy

A hierarchy is an arrangement of items in which the items are represented as being "above," "below," or "at the same level as" one another...

, and subdivided according to similarities and differences.
Ontology, in analytic philosophy, concerns the determining of whether some categories of being

Category of being

In metaphysics , the different kinds or ways of being are called categories of being or simply categories. To investigate the categories of being is to determine the most fundamental and the broadest classes of entities...

are fundamental and asks in what sense the items in those categories can be said to "be".