Major depression also called unipolar depression and clinical depression is manifested by a combination of symptoms that interfere with the ability to work, study, sleep, eat, and enjoy once pleasurable activities. Such a disabling episode of depression may occur only once but more commonly occurs several times in a lifetime.

Dysthymia is a disorder with similar, but longer-lasting and milder symptoms than clinical depression. By the standard psychiatric definition, this disorder lasts for at least two years, but is less disabling than major depression; for example, victims are usually able to go on working and do not need to be hospitalized. Many people with dysthymia also experience major depressive episodes at some time in their lives.

Bipolar Disorder, also called manic-depressive illness is not nearly as prevalent as other forms of depressive disorders. Bipolar disorder is characterized by cycling mood changes: severe highs (mania) and lows (depression). Sometimes the mood switches are dramatic and rapid, but most often they are gradual. When in the depressed cycle, an individual can have any or all of the symptoms of a depressive disorder. When in the manic cycle, the individual may be overactive, overtalkative, and have a great deal of energy. Mania often affects thinking, judgment, and social behavior in ways that cause serious problems and embarrassment. For example, the individual in a manic phase may feel elated, full of grand schemes that might range from unwise business decisions to romantic sprees. Mania, left untreated, may worsen to a psychotic state.