Nicotine: general

Nicotine is the natural alkaloid that makes tobacco addictive [1][2]. It's an addictive stimulant [1], and it's also a toxic chemical in its own right, not just a delivery problem for something else in smoke [3][4]. Both things are true at once: the addiction comes from nicotine, and the toxicity in large doses also comes from nicotine [4].

It works by acting on nicotinic acetylcholine receptors — an agonist at most subtypes, an antagonist at two (nAChRα9 and nAChRα10) [1]. Nicotine is also used as a pharmacological aid to help people stop smoking [5].

The two things worth holding onto before anything else:

Nicotine is what makes tobacco addictive. It's what underlies tobacco addiction and shapes how people end up using tobacco the way they do — the pattern of use, not just the urge [5].

Concentration varies a lot by source. Dried tobacco leaf runs up to about 3% nicotine, commercial cigarette tobacco is lower, around 1.5% by weight, and a less common tobacco species, N. rustica, can run far higher, up to about 14% [2][5].

Nicotine itself is a colourless, oily liquid alkaloid when extracted, and it's also produced by the plant as a natural insecticide [4][5].

References

1. nicotine | Ligand page | IUPHAR/BPS Guide to PHARMACOLOGY — IUPHAR/BPS
2. Nicotine - American Chemical Society — American Chemical Society
3. Nicotine | C10H14N2 | CID 89594 - PubChem — National Center for Biotechnology Information
4. Nicotine - CAMEO — Museum of Fine Arts, Boston
5. Nicotine Chemistry, Metabolism, Kinetics and Biomarkers - PMC — National Institutes of Health