Since you say you know the formal definitions, then I assume you are looking for a common sense understanding.
In my own day-to-day perspective there are things I know because I've experienced them sufficiently (like gravity), or I accept something as "known" because others have experienced them sufficiently. This subject of how we know and what is known is epistomology. Some people believe they know for other reasons than through experience. A few individuals over in the debunking area of PF claim they "know" psychic phenomena do not occur. Do they know, or do they just think they know? In terms of gravity, do we know it operates the same at every single spot in the universe? No, we don't know that even though if we were designing a space ship, we'd assume the law of gravity holds constant. The reason we don't know that is because we can't experience every place in the universe. So I say that knowing (in a healthy mind) is established when sufficient experience establishes certainty; and also, that the reason knowing comes about that way is because it is simply how consciousness operates.
However, besides knowing what the experience of gravity is like, there is also the fact that gravity exists. How is it that gravity exists? What establishes its existence? What is the underlying nature of gravity? All these are ontological questions because they are concerned with the nature of existence.
Finally, while we are dependent on conscious experience to know, gravity itself, for example, has absolutely nothing to do with whether we know it exists or not. So to know is one thing and existence is another. I can't see how one is "versus" another, but from the standpoint of a consciousness desiring to know what exists, of course there is a most intimate relationship between knowing and existence.