Neurotransmitters are chemicals that are used to relay, amplify and modulate signals between a neuron and another cell.
Neurotransmitters are packaged into vesicles that cluster beneath the membrane on the presynaptic side of a synapse, and released into the synaptic cleft, where they bind to receptors located in the membrane on the postsynaptic side of the synapse.
Release of neurotransmitters is most commonly driven by arrival of an action potential at the synapse, but may also be driven by graded electrical potentials. Also, there is often a low level of "baseline" release even in the absence of electrical stimulation.