When a
smoker smokes a
cigarette,
cigar, or other
tobacco implement, s/he inhales a large quantity of
tobacco smoke. That is, presumably, what s/he wants to be doing. Other people around the smoker, however, also end up inhaling some rather smaller quantity of smoke, whether they want to or not. This is termed "
second-hand smoke". A
nonsmoker who is frequently around smokers (for instance, the child of smoker parents, or an employee of a smoky
saloon) is, similarly, termed a "
second-hand smoker".
Smokers seem to see second-hand smoke as an aesthetic issue -- nonsmokers, they believe, simply don't like the smell of tobacco smoke. While this may be true, it is also true that when a person is truly constantly subjected to second-hand smoke, his or her health is significantly harmed. This is especially an issue for children; the rates of asthma among the (nonsmoker) children of smokers, for instance, dwarf those among children of nonsmokers.
If you are a nonsmoker, sitting in the smoking section of a restaurant for a meal with your smoker friends is not going to have a significant effect on your health. However, if you live in a constant fug of tobacco smoke due to your parents or other housemates, you are most certainly being harmed.