Basic ingredients
We all know that tobacco contains nicotine, a drug chemically related to heroin and cocaine. It is more addictive than those two, but gives less pleasure to the addict.
But cigarette smoke contains a lot more poison than nicotine alone. In fact, it contains 4,000 different ingredients, of which 40-50 (depending on the source) are carcinogens (i.e., cancer-causing agents).
Since cigarette packs do not contain a list of ingredients (it would not fit on the pack anyway), here are some of the main ones in alphabetic order, plus brief description.
If you smoke, go ahead and light up before you continue, because any future lightups may not be the same.
And no, I am not mocking you by saying that: I am one of you. I am on my third day without a cigarette, going through withdrawal, and posting this to stay motivated. Trust me, I had no idea of all this crap that I have been inhaling many times a day. In a way I am glad I only searched for and discovered these ingredients two days after I quit!
Basic Ingredients
Cancer Causing Agents
Metals
I based this list on information provided by Massachusetts Department of Public Health, which runs the TryToStop.org web site.
Additionally, Chicago Tribune mentions 25 kinds of alcoholic substances and 55 acids. You can see their list at http://cnews.tribune.com/images/199708/tobacco0826smoke.jpg (yes, it's a picture).