Major depression (or major depressive disorder) is manifested by a combination of symptoms (see symptoms list below) that interferes with the ability to work, study, sleep, eat, and enjoy once pleasurable activities. A major depressive episode may occur only once; but more commonly, several episodes may occur in a lifetime. Chronic major depression may require a person to continue treatment indefinitely.

A less severe type of depression, dysthymia (or dysthymic disorder), involves long lasting, chronic symptoms that do not seriously disable, but keep one from functioning well or feeling good. Many people with dysthymia also experience major depressive episodes at some time in their lives.

Another type of depressive illness is bipolar disorder (or manic depressive illness). Bipolar disorder is characterized by cycling mood changes: severe highs (mania) and lows (depression), often with periods of normal mood in between. Sometimes the mood switches are dramatic and rapid, but usually they are gradual. When in the depressed cycle, an individual can have any or all of the symptoms of depression. When in the manic cycle, the individual may be overactive, over talkative, 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 and unsafe sex. Mania, left untreated, may worsen to a psychotic state.

SOURCE: National Institute of Mental Health