Tobacco smoke containes a number of harmful ingredients such as carbon monoxide, heavy metals, tars and nicotine. Each of these is toxic by itself when inhaled.
The immediate effect from tobacco smoke is the "buzz" associated with nicotine. When ingested, nicotine causes a rapid heart beat, rapid breathing and a feeling of light headdedness. Nicotine is highly addictive, and after a few cigarettes or pouches, one starts "nicking" or feeling cranky and irritable if the nicotine craving is not satisfied.
There are carcinoginic chemicals in cigarette smoke that bind with protiens in DNA causing genetic mutations and birth defects.
In the body, carcinogens from tobacco smoke can bind to blood proteins and to DNA, and can thus produce gene mutations and chromosomal abnormalities. Smoking can also cause changes in the metabolism of cells or tissues, resulting in changes to the way foreign substances are broken down by the body. (http://www.greenfacts.org/en/tobacco/)
Diseases associated with tobacco use are myriad ranging from the most common heart disease and lung cancer to thyroid disease and bladder cancer. Please check out the links below for more detailed information.