Tobacco, an otherwise harmless and in some respects highly nutritious plant, carries a heavy load of nicotine. It is probably there as a natural insecticide. Nicotine is in fact, used as an insecticide. A single drop of pure nicotine is enough to kill you if it is on your skin. Nicotine in your brain directly affects the level of dopamine present at any moment.
Dopamine is the hormone that controls pleasure levels.
It also controls your physical movement. Parkinson's Disease, for example, is a severe deficit of dopamine. So, it is true that nicotine is a brain altering substance because it affects your internal controls of dopamine. You get more dopamine when nicotine is present. This is where hard core addiction takes place.