Well, incotine in tobacco, like amphetamines, it is sympathomimetic. Thus, it substitutes itself into neuroreceptors that stimulate the sympathetic nervous system(fight or flight, e.g. responsible for heart rate speeding up, sweat production increase). It increases concentration and relieves tension and its effects include increased blood pressure.
I heard from my biology teacher that the body has specific nicotine neuroreceptors to allow nicotine bonding. That may be one of the factors involved in why humans are so addicted to this drug.