In philosophy the *(term) ontology is used to indicate a theory of being, which describes "what is" and in some cases by converse "what is not", and the relationships between beings. One of the simplest ontologies is Parmenides': existence is - stop. This rules out distinction among different things, and therefore space, time, quality, yielding a reality devoided of any informative content since a system with only one state can be encoded with zero _(bit)s. Although Parmenides' approach raised many interesting philosophical issues at the time it was introduced (the well-known Parmenides' paradoxes) we want more complex ontologies both for theoretical and for practical purposes. 

An ontology is also the basic vocabulary to start speaking - and reasoning - about anything. As such, communication cannot occur unless the speakers agree on a common ontology, an issue known as ontological commitment. To commit on a common ontology, however, speakers must communicate -- and therefore they must already be both committed to a (perhaps simpler) common ontology, a typical _(bootstrap) problem. Computer scientists  ontologies are commonly meant as organized vocabularies to make distributed computing systems interact. Researchers in knowledge representation are usually more precise. A definition from Tom Gruber's "What is an ontology?" (see links below):
<p><cite>
In the context of knowledge sharing, I use the _(term) ontology to mean a  specification of a conceptualization. That is, an ontology is a description (like a formal specification of a program) of the concepts and relationships that can exist for an agent or a community of agents. 
</cite></p>

<ul class="links">
<li>_(What is an ontology?| http://www-ksl.stanford.edu/kst/what-is-an-ontology.html) From Tom Gruber, Stanford Knowledge System Laboratory.</li>
<li> _(Knowledge representation at Mizoguchi lab| http://www.ei.sanken.osaka-u.ac.jp/). Comprehensive.</li>
<li> _(Knowledge Sharing Public Library| http://ontolingua.stanford.edu) from Stanford University. An example of ontology. A public ontology navigator is available: Log in anonymously for a read-only access. In KIF (Knowledge Interchange Format) language.</li>
<li>The _(semantic web| http://www.semanticweb.org/) homepage.</li>
<li>The standardization effort at _(w3 | http://www.w3.org/2001/sw/WebOnt/).</li>
</ul>

<hr />

<span class="comment">Desperately needs improvement, but could be a starter</span> -- _(schizophonic)