Neurotransmitters are chemicals. They transmit information between different neurons by crossing a synapse. This is because the electrical signals are not able to cross the gap between most neurons and so are changed into chemical signals in order to cross the gap. Neurotransmitters act mostly on chemical synapses. Once they reach the next neuron they are absorbed. The neuron then changes this chemical signal back into an electrical signal called an action potential. The action potential passes across the next neuron and to the next synapse.
Each neurotransmitter has a different function. For example: Dopamine is used in reward and pleasure and Noradrenaline is used in an animal's "fight or flight" response. Neurotransmitters also regulate the passing of messages. This is because an action potential must be a certain strength before the neurotransmitters are released. The strength required to release the neurotransmitter is called a threshold.
Neurotransmitters are transported within neurons by small "sacks" called vesicles. When these vesciles come into contact with the neuron's cell membrane, it opens. This releases the neurotransmitters into the synaptic cleft. The vesicle is opened through a process involving SNARE peptides.