What is DNA, really?

If you could unwind all the DNA from a single one of your cells and stretch it out, it would be about two metres long, taller than most adults. And you have trillions of cells. Stretched end to end, the DNA inside you could reach far beyond the Moon and back, many times over. All of that, coiled up invisibly small inside you right now. So what exactly is this stuff, and why is there so much of it?