Chemistry in cigarettes
Cigarettes and other forms of tobacco consist of dried tobacco leaves, and other ingredients added for flavor and other properties.

Why smoking causes cancer? It's because tobacco and tobacco smoke contain more than 60 carcinogenic compounds. In general, more than 4,000 individual substances have been identified in tobacco smoke, including carbon monoxide, hydrogen cyanide, ammonia and other highly toxic irritants. Besides all the harmful effects of tobacco, it is addictive, and this explains why although 70% of smokers want to quit and 35% attempt to quit each year, fewer than 7% succeed. And the main reason why tobacco becomes addictive is due to its content of nicotine, which alters brain functioning.
Nicotine is a naturally occurring liquid alkaloid. An alkaloid is an organic compound made out of carbon, hydrogen, nitrogen and sometimes oxygen. These chemicals have potent effects on the human body. For example, many people enjoy the stimulating effects of another alkaloid, caffeine.
Nicotine:

When you smoke, nicotine is absorbed through the skin and mucosal lining of the mouth and nose or by inhalation in the lungs. Once in the body, it activates the same reward system as do other drugs such as cocaine or amphetamine, although to a lesser degree. In the brain, nicotine increases the level of the neurotransmitter dopamine, which is a chemical in the brain responsible for feelings of pleasure. The acute effects of nicotine subside within minutes, so people continue dosing themselves frequently throughout the day to maintain the pleasurable effects of nicotine and to prevent withdrawal symptoms.