Short answers, pulled from the story.
Ontology is the philosophical study of being. It is traditionally treated as the subdiscipline of metaphysics focused on the most general features of reality, examining what all entities have in common and how they divide into basic categories such as particulars and universals.
Particulars are unique, non-repeatable entities, such as Socrates, the Taj Mahal, and Mars. Universals are general, repeatable entities, such as the color green, the form circularity, and the virtue courage, which express features shared by particulars.
Concrete objects exist in space and time and carry causal powers, such as a tree, a car, or a planet. Abstract objects, like the number 7 and the set of integers, lie outside space and time, lack causal powers, and do not change.
The word ontology was coined by philosophers in the 17th century. The ancient Greeks speculated about being but never used the term, even though its roots trace to ancient Greek words for being and study.
Platonic realism asserts that universals have objective, mind-independent existence, even apart from the objects that exemplify them. Conceptualism holds that universals exist only in the mind as concepts, while nominalism denies that universals exist at all, leaving a world of particular objects.
In computer and information science, applied ontology builds formal frameworks to encode and store information about a limited domain using categories like person, company, address, and name. Upper ontologies such as the Suggested Upper Merged Ontology and Basic Formal Ontology let databases from different domains be integrated, and frameworks like Gene Ontology and Protein Ontology standardize biological information.