The core of MetaPhysics, ontology is the study of existence in general, or in AristotlE's memorable phrase, the study of BeingQuaBeing?. So ontology is the study of existing things insofar as they exist, or as to the aspect of ExistEnce.
While perhaps there's not any one thing such obscure phrases must mean, what they have come to mean, due to AristotlEs influence, is the study of the most basic CategoriesOfBeing?--and of "NonBeing?" as well. (MeinonG?, who thought we can talk unobjectionably about nonexistent objects such as the golden mountain, was an ontologist.) A "category of being" is a type or class of thing--ThinG?'' being used in its broadest possible sense--that cannot be reduced to (see ReductioN) or explicated (see ExplicatioN?) in terms of any other category. Among the many and varied categories that have been postulated over the millennia, some perennial favorites are ObjectS? (presumably, PhysicalObjects?), UniversalS?, PropertieS?, RelationS?, EventS?, TheMind? (or something mental), and SeT's.
I made a little box in the SandBox
I can't make a box :( Is it a Mac thing? I can copy and paste boxes, though.