In the tobacco plant, nicotine is synthesized in the roots of the plant and then stored in its leaves. The purpose of this chemical is as a repellent. Nicotine serves to discourage herbivores from eating the tobacco plant, especially insects. For a while, nicotine was even used as an insect repellent because of this property.
Nicotine is an insect repellent
Tobacco is an alkaloid chemical, similar to caffeine. Because of its properties as an alkaloid, nicotine can serve as a stimulant to mammals in small quantities. Higher doses can sedate or kill an animal. Nicotine burns at a lower temperature than it's boiling point, this is how the nicotine in a cigarette becomes a vapor that you inhale. This vapor has a fairly low combustion temperature however, around 308K (35 C), so most of the vaporized nicotine is burned up before it gets inhaled.
Most of the nicotine is being burned away.
This post is more about nicotine itself, but I would be remiss if I did not cover the health concerns associated with smoking and various tobacco related habits. Nicotine is a highly addictive substance. It can diffuse into your blood stream through your lungs, skin and mucus membranes. Once the chemical has entered your body, it can reach your brain in about 10 to 20 seconds. When nicotine reaches your brain, it triggers the release of dopamine, as well as other chemicals like adrenaline. Dopamine is what causes addictions. Dopamine is a form of positive feedback for the brain, it makes you feel good, and tells your body that what you just did was something good. This is essentially the same process that makes us eat, so it can become highly addictive.
Nicotine triggers the release of dopamine
As I mentioned, nicotine also triggers the release of adrenaline, which can invigorate the smoker. This adrenaline also triggers the release of glucose into the blood stream which provides energy, and can reduce the apetite of the smoker.
The harm from smoking doesn't just come from the smoke alone, nicotine also has a series of other negative effects on mammals, other than creating an addiction. Nicotine in the blood system can actually harden the arteries, causing severe heart disease. No matter how good it feels, smoking, taking snuff or chewing tobacco will all eventually kill you. Don't do it kids.