The act of inhaling smoke into the lungs and eventually exhaling (can be done through the mouth, nose, and ocassionally directly below the eye). Most commonly, people choose to smoke cigarettes, which are primarily made from the leaves of the tobacco plant. Commercially produced cigarettes also contain a massive amount of chemicals.
CAUTION: smoking cigarettes seriously damages your health. Known effects include cancers of the oesophagus, larynx, mouth and lungs. Also, smoking destroys the epithelial lining of the trachea (wind-pipe) which results in an inability to shift mucus which builds up there. This causes clogging of the airways, resulting in "smoker's cough" and increased risk of lung infections (e.g. pneumonia, tuberculosis etc.), asthma, hypoxia and other widespread diseases.
Another commonly "smoked" substance is marijuana, which contains the active ingredient of tetrahydracannabinol (THC). Smoking marijuana will get you high/stoned/baked and can make a person feel happy and euphoric for a period of hours. Ocassionally, certain people can and will get stoned enough to make themselves sick. Marijuana can also be added to brownies with interesting results.