Neurotransmitters are chemicals that transmit signals between neurons. The document discusses several key neurotransmitters like acetylcholine, norepinephrine, dopamine, GABA, glutamate, and serotonin. It describes their functions and roles in processes like memory, mood, movement, and sleep. The document also outlines the criteria for classifying a chemical as a neurotransmitter and how neurotransmitters are synthesized, released, bind to receptors, and degraded once their signaling work is complete.