An ontology is simply a description of a small range of concepts and relationships . The best way to expound is to give an example of one in use:
Simple, n'eh? The ontology here includes what "is a kind of" means, what "used in" means, what alternative sorts of wine there are ("white" and "rose", for instance). It might also include analogies to other ontologies (my "red wine" is his "vino tinto"), and possible deductions (if Merlot is a kind of red wine, and red wine is a kind of wine, then Merlot is a kind of wine).
There are several steps in teaching a computer an ontology. OWL lets you teach the computer these steps once, and then write many a new ontology without ever leaving the ResourceDescriptionFramework again. Examples of OWL in use: