Acetylcholine is a biochemical and functions as a neurotransmitter. It is created in your brain by Cholinergic neurons but must attach to a receptor site in order to carry a signal. Acetylcholine carries tiny electrical signals through the space in between neurons, known as a synapse. It is also a transportation vessel for signals traveling from the brain to a muscle cell, where it triggers muscle contractions.
There are two types of acetylcholine receptors, nicotinic and muscarinic. While both react to Acetylcholine, the nicotinic receptors, as you would expect, react to nicotine, the common chemical that attributes to cigarettes’ addictiveness.
Acetylcholine also has a large hand in regulating memory in the brain, and can be synthesized from Choline, and supplementing with Choline can help induce this reaction along with other chemicals.
If you are looking for more information on Acetylcholine, you can download a .pdf that goes into great detail about the chemical and its functions HERE.