By the W3C the term ontology is defined as following:

Ontology is a term borrowed from philosophy that refers to the science of describing the kinds of entities in the world and how they are related.

If you start to dig a bit you’ll realize a couple disillusioning facts:

The Web Ontology Language is there for quite a while. But there is no significant impact on Web Engineering, yet. RDF and thus OWL is not designed to be read by humans. But OWL is not supported out of the box, e.g. in the .NET framework. To design ontologies you would need an editor, but all you can find is Protégé. Protégé is not bad at all, but the Protégé OWL documentation roughly four years old. The only elaborate documentation on OWL is the W3C recommendation.