Neurotransmitters are released from vesicles in axon terminals to exert an effect on the dendrites of the postsynaptic neuron or effector muscle/organ. The effect can be either excitatory or inhibitory, depending on the neurotransmitter and the receptors on the affected neuron/muscle/organ. Furthermore, neurotransmitters can either work directly by binding to receptors on ion channels, or they can work indirectly via a 2nd messenger system. When their task in complete, they are removed from the synapse in 3 potential ways: diffusion, degraded by enzymes, or reuptake pumps in the presynaptic axon terminals.
Click the link below for information on several of the most common neurotransmitters.