Serotonin
A chemical produced by the brain that functions as a neurotransmitter. Low serotonin levels are associated with mood disorders, particularly depression. Medications known as selective serotonin reuptake inhibitors (SSRIs--ie Prozac) are used to treat disorders characterized by depressed mood.

Catecholamine: any of a group of sympathomimetic amines (including dopamine, epinephrine, and norepinephrine) which function as neurotransmitters in the brain
The catecholamines play an important role in the body's physiological response to stress ( the "fight or flight" response to perceived severe stress/danger--these neurotransmitters are responsible for the reactions one gets such as increased heart rate, increased respiratory rate, dilated pupils, increased perfusion to muscles, etc.
One of the major catecholamine uptake inhibitors (theoretically helps keep the neurotransmitters around longer, stopping their degradation) is Welbutrin-


Thus, Prozac works on decreasing serotonin degradation, keeping it around longer; Welbutrin works on the catecholamines norepinephrine and dopamine, keeping them around longer....

http://medical-dictionary.thefreedic.../catecholamine