Serotonin (5-hydroxytryptamine, 5-HT) produces its effects through a variety of membrane-bound receptors. 5-HT and its receptors are found both in the central and peripheral nervous system (CNS/PNS), as well as in a number of nonneuronal tissues in the gut, cardiovascular system and blood. In evolutionary terms, 5-HT is one of the oldest neurotransmitters and has been implicated in the aetiology of numerous disease states, including depression, anxiety, social phobia, schizophrenia, and obsessive–compulsive and panic disorders; in addition to migraine, hypertension, pulmonary hypertension, eating disorders, vomiting and, more recently, irritable bowel syndrome (IBS). 
5-HT is probably unique among the monoamines in that its effects are subserved by as many as 13 distinct heptahelical, G-protein-coupled receptors (GPCRs) and one (presumably a family of) ligand-gated ion channel(s). These receptors are divided into seven distinct classes (5-HT1 to 5-HT7) largely on the basis of their structural and operational characteristics.Not surprisingly, the 5-HT receptor family has been a long-standing target of intense research, in both the academia and the pharmaceutical industry, even before the complexity of the system was unravelled by molecular cloning.

References:

1.Hoyer D, et al. Pharmacol Biochem Behav. 2002 Apr;71(4):533-54.