Smoking affects nearly every organ of your body and can cause cancer anywhere in the body. Compared to non-smokers, smokers are 25 times more likely to develop lung cancer and have a 2 to 4 times increased risk for coronary heart disease and stroke. Smoking causes 90% of all lung cancer deaths and 80% of all death from chronic obstructive pulmonary disease (COPD). Smoking can also cause harm to the unborn, such as preterm delivery and stillbirth.

Does reducing the number of cigarettes smoked per day but not stopping lower your risk of health hazards?
A study from Norway showed that there is no reduction in premature death in patients who significantly reduced but did not quit smoking. By contrast, quitting entirely, at any age, significantly increases life expectancy.
Source: Tverdal, A., & Bjartveit, K. (2006). Health consequences of reduced daily cigarette consumption. Tobacco Control, 15(6), 472-480.