Quite a lot could be written on the subject of existence. Unlike some of the esoterica on this site, everyone here presumably has some experience and could write about what existence is. Such discussions could easily get into some murky territory, even our respected competition, usually known for their endearing allegiance to "factuality" seems to be unable to keep the discussion on the topic of existence within the straight-and-narrow.
So, I am going to confine my discussion of existence to one facet, and am in a way almost using existence as just one example of another issue, the issue of constructivism. The more I learn about both education and philosophy, the more I appreciate the position that the world is not "just there" but is instead a result of conceptual filters on the world, caused by anything from neurological patterns to social indoctrination. I am also surprised how many naive realists there are, even amongst people who should know better. Of course, constructivism is not an easy position to convince people of, especially in its bastardized forms. You can not actually make the world go away or change forms by closing your ears and humming. So what is the most basic example of constructionism? Is there an example that gets to the basics of it, the way that KRS-One phrased the question of justice in America by asking "Can there really ever be justice on stolen land?"
Existence to me is a basic example of a concept (as the above poster phrased it), or better yet, a category. Existence is an in-built category that our minds filter the universe through. If this seems like an odd thing to say, take a look at about five objects in front of you and ask where there existence lays. Is the existence inside of them, like a tootsie roll inside a tootsie pop?. Could you dig apart objects and find inside of them, little particles of existence? And then, what would be left of the husk? Would it exist or not? Various and sundry objects around us are labeled as "existing", but what do these disparate objects have in common? The only answer really is that they can somehow interact with each other, and yet we can at least imagine an object that is unable to interact with other things yet still exists. So then, it is hard to say what the things that share this common concept "existence" really have in common. When investigated, the taken for granted concept of "existence" makes no more sense than the social concepts that make us live the way that we do.
As I said, I don't mean that just because "existence" is a concept that it isn't real. I just mean to say that once that concept is questioned, any other concept can be very easily questioned.