Nicotine is a naturally occurring chemical compound, and like many alkaloids, have a physiological effect on humans and other animals.
Found organically in the nightshade plant family (Nicotiana tabacum, Asclepias syriaca, Duboisia hopwoodii, and Nicotiana rustica), it is a parasympathomimetic drug.
For the curious, this means that it stimulates the parasympathetic nervous system. This part of the nervous system plays a role in processes like digestion and sex.
Want to know more about how nicotine affects the human body? Check out this article: The Healthier Alternative to Smoking