There are at least 250 varieties of chemicals in cigarettes, most of them toxic- and at least 50 of them are cancer-causing or cancer-aggravating.
Here is a brief summary:
Cigarettes contain carcinogens, which means they can cause or aggravate cancer.
Cigarettes contain toxic metals, such as Arsenic and Cadmium (typically used in batteries).
Cigarettes contain the poisons Ammonia, Carbon Monoxide, Hydrogen Cyanide, and Nicotine.
Keep in mind that these chemicals are also present in secondhand smoke- and that, according to the U.S. Surgeon General, all exposure to secondhand smoke is risky.