Neurotransmitters are chemicals released from the axon terminal of a sending neuron into the synaptic cleft where they then bind to a receiving neuron instructing the cell to fire or not fire. Neurotransmitters are excitatory or inhibitory.
There are many neurotransmitters involved with higher-level brain functions such as euphoria, sleep initiation, and social behavior. Here are some that fall under the monoamine classification:
acetylcholine - learning, movement, memory
norepinephrine - social behavior, eating, learning, memory
dopamine - emotional behavior, attention, learning, addiction, pleasure, euphoria
serotonin - sleep, arousal, depression
endorphins - pain reduction, learning, emotional behavior
glutamate - learning