A neurotransmitter believed to regulate mood. SSRI's inhibit the reuptake of serotonin in the brain (leading there to be more serotonin left as a result), hopefully improving the mood.
Personally, a good blast on the Codeine cheers me up more than any SSRI.
Perhaps it's my endorphine system rather than my serotoninergic system that is fucked.
A lack of Serotonin is believed to be a cause of depression. It also controls appetite, among other things.
Antidepressants often (through various mechanisms) increase the amount of serotonin in the brain.
MDMA (Ecstacy) causes a massive (cascading) release of serotonin in the brain - responsible for both it's euphoric effects, and the depressive aftereffects (less serotonin remains in the brain, until generated again)