types of depression :
major depressive disorder : this disorder is characterized by a depressed mood or loss of interest in pleasure for at least two weeks.
dysthymia : this disorder is characterized by a mild or moderate chronic state of depression for at least two years (or at least 1 year in children and teenagers).
seasonal affective disorder : also known as sad, this disorder is a kind of major depression which occurs during the fall and winter months, caused by the lack of sunlight, which helps the brain produce melatonin. melatonin is a chemical in the brain that controls the body's sleeping and eating habits.
bipolar or manic-depression : this disorder is characterized by extreme "mood swings", a cycle between depression and mania. mania is characterized by feeling overly elated, irritable, unusually energetic or inability to sleep, trouble staying focused, and making decisions or acting in a way that will later be regretted. each phase, clinically, is said to last at least one week.