The major active chemical in tobacco is Nicotine. Consumed usually through smoking cigarettes, but also (in order of popularity) as cigars, smokeless tobacco such as chew, and more recently snuz, and finally smoked as pipe tobacco.
Nicotine is by far the most consumed drug in the US, with 46.5 million people within the ranks of the smokers. However, since 1965 the amount of smokers in the US has dropped from 42% to 20.8% in 2006.
A good sign given the plethora of negative health effects associated with tobacco, which include a great negative performance in lung and heart health, significantly adding risk of heart attack, stroke, chronic obstructive pulmonary disease, emphysema and a variety of cancers, including (but not limited to) lung cancer, mouth cancer, and pancreatic cancer.
There are the reasons which lead to the fact that 443,000 people within the US die every year due to smoking. We recommend that you not smoke.