Aristotle was our first famous ontology builder. Here is a tiny one he built: (a)Category: Men; (b)Member of category of Men: Socrates (c)Rule for category of Men: All men are mortal.
This ontology summarizes one fact about the category of men, and permits a deduction about its members, to wit:
Socrates is mortal.
The ontologies I'm familiar with look like more elaborate versions of this: A semi-hierarchical set of categories, rules relating the categories, rules for assigning membership, and a deductive system to determine implicit relationships.
A board game like Clue requires building a mental ontology based on a few categories like location, suspect, and murder weapon. One then accumulates evidence that permits deducing specifics.