Ontology consists of the philosophical study of existence, beingness or the state of being, and reality.
Ontological study is traditionally considered a part of the metaphysical category of philosophy. This study seeks to answer questions about how entities exist and how they can be grouped, placed in a hierarchy, and subdivided into categories such as orders, phylums, and species, etc.
For instance, if you are looking at animals that swim you are dealing with fish (the majority of swimming animals). But there are also swimming mammals (whales, porpoises, and dolphins) and birds that can swim (ducks and penguins). Each of these species is categorized by physiology, diet, natural abilities, method of respiration, anatomy, etc. in order to find its place on an ontological chart.