Smoking is addictive because of the nicotine in tobacco that induces a physical craving. It is also habituating as a social activity, because it gives people an activity with hands and mouth and a "smokescreen" to hide behind in social interactions. It suppresses appetite and destroys sense of smell, reducing enjoyment and interest in food; this is especially seen as a benfit by women who wish to lose weight (hence the marketing pitch as "Virginia Slims").
The respiratory tract is lined by cells that have tiny hairs (cilia) on their surfaces. The cilia beat in waves, moving a carpet of mucus up from the respiratory bronchioles to carry out dust and particles that settle there. Smoking a single cigarette paralyses them for hours, so that the mucus collects in gobs in the lower respiratory tract and must be coughed out ("smoker's cough"). Repeated smoking destroys the cilia altogether, and stimulates the production of mucus which, because it lies stagnant, often becomes infected as chronic (long-term) bronchitis. The muco-pus that results is often yellowish or greenish, coloured by pigments produced by the bacteria that grow in it and the products of the pus cells that they attract. This chronic infection destroys elastic tissue in the lungs, leading to emphysema.
The basis of the cardiovascular disease associated with smoking is less clear-cut. Smoking damages elastic tissue in arteries, as in other parts of the body. It provokes blood clotting. It contributes to oxidation of lipoproteins, increasing the deposition of lipid into arteries.
Smoking cessation is a major strategy for reducing cardiovascular risk. Those who smoke five or fewer cigarettes per day are unlikely to suffer physical withdrawal from nicotine, and nicotine replacement therapy is not helpful to them. Those who smoke more may be helped by replacement therapy, which is now available with government subsidy through the QUITLINE programme.
The social aspects of smoking are more dificult to treat. It helps if your smoking spouse, partner or flat-mate is willing to give up in company with you. It helps to avoid places (like the pub or club) where smoking is part of the routine. It helps to find some other activity to keep your hands busy -- maybe a worry egg or worry beads. It helps to start some new activity (sport, gym) that is incompatible with smoking.
Because smoking suppresses appetite and the enjoyment of food, many who stop smoking find they put on weight. Be aware of that, and watch your weight carefully. It is MUCH harder to take weight off than to avoid putting it on.