Ontology is a branch of metaphysics that asks what sort of things exist. For wittgenstein, what exist is limited to objects and states of affairs existing in a logical space.
Ontology is a kind of human logic that is different from 'factual knowledge as we generally understand it' (which is called ontics) . For example, ontically, I put away money into savings because the old man who draws it will be the same person as I am now (this is a fact) . But the same scene viewed ontologically, focuses on the fact that the old man will only be the same person as I am now because of the fact that I am saving for his pension (this second viewpoint concerns the almost metaphysical question
Ontology is concerned with being and its understanding. Thus, dasein is distinguished ontically (factually) in that it alone among beings is ontological. Which is to say, it alone understands the being of beings.
Ontology is an interest not in how particular entities came about but in the mind-bending fact that there are entities in the first place.
Ontology is a departure from Husserl and the cartesian tradition in that it substitutes for epistemological questions concerning the relation of the knower and the known, 'ontological' questions concerning what sort of beings we are and how our being is bound up with the intelligibility of the world.
Ontology is concerned with the idea that our transparent everyday way of coping with the available can be carried on independently of the emergence of a thematically conscious subject.
Ontology is concerned with the fact that reality rests on a fundamental disjunciton whereby it is not that our ideas about reality do not line up with reality. Rather, the problem is that reality does not line up with itself.