Ontology deals with entities and how they can be categorized and organized according to their attributes. This includes how metadata is modelled and interpreted.
Ontology deals with questions concerning what entities exist or can be said to exist, and how such entities can be grouped, related within a hierarchy, and subdivided according to similarities and differences (see also Wikipedia). Ontology is the branch of metaphysics that studies the nature of existence or being as such.
In the context of databases, ontology deals with metadata and schemata. Questions with this tag deal with the proper modelling and interpretation of metadata.