Despite popular belief, chiropractic is much more than the "cracking of bones". Chiropractic is actually about the function of your nervous system, which controls all the processes within your body. When a chiropractor adjusts your spine they are trying to remove interference from within your nervous system in order to allow your body to function better.
Below is a video that sums it all up, and it's a lot more interesting than listening to me babble on. If you've got a spare 10 minutes on your hands go and make yourself a cup of tea and check it out.