The dependance is physical and psychological. It is essentially the nicotine found in tobacco that creates this physical dependance. As a smoker, the brain is used to certain level of nicotine. If the brain doesn't get the nicotine it's used to, the following nicotine withdrawal symptoms may appear:
Nicotine Replacement Therapy (NRT) can help alleviate the withdrawal symptoms and you can gradually reduce your nicotine intake and get your body used to the change.
NRT comes in various forms and strengths to help you stop smoking. You now have Nicotine patches, Nicotine Gum, Nicotine Lozenges, Nicotine Inhalators, Nicotine Spray, Nicotine Microtabs. There's a big range of NRT products to help all sorts of smokers stop smoking.