What is Ontology?
Quote from Wikipedia "is the philosophical study of the nature of being, existence or reality in general, as well as the basic categories of being and their relations....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"
Quote from other web-site "In ancient times, Ontology referred to study of Being and fell into disrepute in modern times, but Edmund Husserl revived the idea of the study of different forms of Being and Phenomenology and Existentialism have given rise to a new concern with Ontology, the central problem of which is the age-old problem of Mind and Body."
Quote from Zachman web site "a theory of the existence of a structured set of essential components of an object for which explicit expressions is necessary and perhaps even mandatory for creating, operating, and changing the object"
What is Typology and Taxonomy then?
I fund so far the best explaination is from the web site http://www.marxists.org/glossary/terms/t/a.htm:
Taxonomy and Typology are two opposed methods of classification. Typology classifies according to the sensuously given attributes of the thing; Taxonomy classifies objects according to the genesis of the thing.
At a given stage in the development of a science, when as yet the genesis of the object is not known, typology is the only possible method of classification. Classification into types and classes aids sensuous perception. Once a notion of the genesis of the object is gained, the types must be re-ordered and objects which appear to belong to the same type, fall into different species and genre of the new taxonomy and vice versa, objects of different type are found to be of the same species.
Typology and taxonomy are necessary stages in making sense of the world, which aims to revealing the inner connections between things.
My Conclution:
Typology and Taxonomy are the types of Ontology. You usually start with Typology by observing the world and you can only confirm that your Typology is a Taxonomy when most of people conclude that it's true.
Analyzing the objects and classify them in a Taxonomy is always the first step we build and operate on their 'right' perceptions - the architecture of a enterprise system and even a software system.
This is also the way called 'frame it' in the book 'THE MCKENZIE WAY'.