The human body is an amazing machine.  No matter how much you abuse it with crappy food, lack of exercise, and a general beating it always manages to try to heal itself.  Nowhere is this more evident in relation to smoking.  Cigarette smoking is one of the most damaging things you can do to your body as it scars your lungs, restricts your breathing, affects your arteries and restricts heart function.  With all that said, as soon as you stop smoking your body immediately begins to repair itself.

For those quitting "cold turkey" the chart below (courtesy of WhyQuit.com) shows the typical progression of healing that a body goes through.  Each person is different, but these guidelines should give you hope if (and when) you decide to give up smoking.