There are around 4000 chemicals in cigarettes. The one you hear of the most is nicotine, the chemical that makes people want keep smoking and makes it very hard to quit smoking. Other chemicals found in cigarette tobacco are such things as formaldehyde (which is the stuff that you preserve bodies in when people or animals die), acetone (which is the stuff that you remove nail polish with), acetic acid (vinegar), ammonia (stuff that you clean the floors with), arsenic (stuff in rat poison), carbon monoxide (the stuff that comes out of a car tail pipe), toluene (an industrial solvent, also found in gasoline), as well as several metal such as mercury (which made the Mad Hatter mad) and lead (which can lead to learning disabilities), and many, many others that I can't pronounce the name of! In addition, a majority of these chemicals have been found to cause cancer in humans.

Cigarettes contain nicotine, a chemical poison that the tobbacco plant makes to prevent insects from eating the plant. It turns out that nicotine binds to the surface of nerve cells in the brain of humans. Upon binding, it stimulates the nerve cell and gives the smoker an "awake", "buzzy" feeling (according to smokers). The nicotine is addictive and makes the smoker crave more cigarettes.
Cigarette smoke also contains tar - a substance much like the black goop they put on the roadways. Tar causes cancer. This gooey tar coats the lungs, turning them black. The tar is a poison to the cells in the lungs. The tar chemicals, called polycyclic aromatic hydrocarbons, get into the lung cells and cause mutations in the DNA of the cell. Mutations are chemical changes in the DNA that cause the wrong proteins to be made by the cell. Sometimes, the cell will change from a non-growing state to a rapidly growing state when it is mutated. The cell grows out of control, divides, and multiplies, eventually forming a lump of cells, called a tumor. Cells break off from the tumor and travel throughout the body and form more tumors. Eventually the tumors will disrupt some vital system - like the brain or the heart or the lung, and the smoker will die.
For years, cigarette makers claimed there was no scientific evidence that cigarettes caused cancer. But the connection has been supported by years of scientific study. Finally last month on company that makes cigarettes admitted that smoking causes cancer.

Some of the poisons in cigarette smoke are:
carbon monoxide
nicotine
ammonia

Suspected and known carcinogens:
benzene
isoprene
toluene
2-butanone
styrene
benzopyrene
catechol
various nitrosamine

Suspended particulates that contain tars and other carcinogens