What's the difference between taxonomy and ontology?

From WhyNotWiki

http://en.wiktionary.org/wiki/Ontology. Retrieved on 2007-05-11 11:18.

1. (Philosophy) The branch of metaphysics that addresses the nature or essential characteristics of existence and of things that exist.
2. (Computer Science) A structure of concepts or entities within a domain, organized by relationships; a system model.

http://en.wiktionary.org/wiki/Taxonomy. Retrieved on 2007-05-11 11:18.

1. (systematics, uncountable): The science of finding, describing, classifying and naming organisms.
2. The classification in a hierarchical system.

So it looks like a taxonomy refers strictly to hierarchical systems (whose only relationships are 'child of' and 'parent of', whereas an ontology is a bit more free, allowing any type of relationship to be expressed.

What they have in common: Both, however, are used to add structure (and "semantic value"?) to information in a domain.

Can both be said to have to do with "metadata"?

Can it be said that taxonomy does not have to do with modeling a system?

Are both useful?

Can it be said that they are quite different or should I consider them "pretty similar"?

taxonomy	classification	hierarchy
ontology	modeling, relationships	triples, relations