The major neurotransmitter systems are the noradrenaline (norepinephrine) system, the dopamine system, the serotonin system, and the cholinergic system. Drugs targeting the neurotransmitter of such systems affect the whole system, which explains the mode of action of many drugs.
Most other neurotransmitters, on the other hand, e.g. glutamate, GABA and glycine, are used very generally throughout the central nervous system.