There are many answers to this question, ranging from the philosophical to the scientific, but here's an intriguing new possibility: We might all be nothing more than a big bunch of microscopic black holes. Unless you're a science nerd this might seem a little weird, but bear with me.
Although we usually think of black holes as gigantic things that suck up stars and even whole galaxies, they can actually be any size. And black holes "evaporate" by giving off energy and getting smaller, so there could be lots and lots of really tiny black holes floating around. So what would all those microscopic black holes look like?
That's the sort of question physicists love, and a couple of them recently made a very interesting discovery. Turns out that when you apply the rules of quantum mechanics to tiny black holes the result looks exactly like elementary particles, the stuff atoms, and therefore you and me, are made of. And quantum mechanics tells us that things which look the same are the same (I'm simplifying this just a bit here). Anyway, the implication is that all matter may be made of microscopic black holes. How cool is that!
Of course, even if we are made of microscopic black holes it doesn't really change anything. The value of this discovery is that it may open up a revolutionary new way to look at unified field theory and the very early history of our universe, two of the greatest mysteries of modern physics.
Here's the original paper (not exactly light reading): Quantization of Black Holes in the Shielded Strong Gravity Scenario (I. Neutral Scalar States)
(Black hole image from Wikipedia, originally created by NASA)