Neurotransmitters are chemical messengers that transmit signals from a nerve cell. The signals can be passed to other nerve cells, muscles, or glands. The brain uses them to keep homeostasis (breathing, digestion, heart rate, etc.) They also effect things like sleep cycles and one's mood. When a nerve cell fires a nerve impulse, nerve transmitters are packed inside synaptic vesicles. The vesicles fuse with the cell membrane. This releases the neurotransmitters. They cross a fluid filled gap. The gap is called the synaptic cleft. The neurotransmitters land on receptor sites of another cell and pass on the impulse.
There are many types of neurotransmitters. For example, acetocholine activates muscles.