The American Psychiatric Association (APA) recognizes many different types of depression. Each of these disorders is defined and their symptoms are described in the DSM-IV - the Diagnostic and Statistical Manual of Mental Disorders - published by the APA.

Major Depressive Disorder (or “major depression”) refers to what’s commonly called “depression” in layman’s terms. A person with major depressive disorder has a depressed mood and/or a loss of interest or pleasure in daily activities for at least two weeks, and often much longer. This change in mood negatively impacts the person’s functioning at work, in family life, at school, and/or socially. Some people only experience a single depressive episode from which they recover and regain their positive outlook. Others struggle with recurrent depression and experience many periods of depression throughout their life. It’s estimated that 10-25% of women and 5-12% of men experience major depression at some point during their lifetime. At any given time, 5-9% of the female population and 2-3% of the male population are struggling with major depression. (See the DSM-IV criteria for major depressive disorder.)
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Dysthymic Disorder (or “dysthymia”) is milder but longer-lasting form of depression. People with dysthymia find little joy or excitement in life - but instead feel bored, worried, sluggish, and/or irritable - for long stretches of time. Unlike a major depressive episode, which although severe may only last for weeks or months, dysthymia can persist for years (or even decades in some cases). Adults that developed dysthymia early in life may believe that it’s normal to feel depressed all the time, and as a result they often do not recognize the disorder or take steps to treat it. Dysthymia affects about 3% of the population at any given time, and about 6% of all people will experience dysthymia at some point in their life.

Bipolar Disorder, also known as manic depression, is characterized by extreme mood swings alternating between depression and mania. During the depressed phase, a person with bipolar disorder can experience the symptoms associated with major depression. During the manic phase, however, they experience an elevated mood that can include excessive energy, elation, euphoria, racing thoughts, little need for sleep, impulsivity, overconfidence, and feelings of importance or grandeur. Although mania can be enjoyable, it can also be a destructive force that impairs judgment, causes feelings of being overwhelmed or out-of-control, and leads to inappropriate or reckless behavior such as drug use, spending sprees, or promiscuous sex. In the extreme form, a manic person may lose touch reality and even experience hallucinations. The duration of the manic and depressed phases can vary, with some people cycling between the two in a matter of weeks and others remaining depressed or manic for years. In general, the depression experienced by people with bipolar disorder can be severe, and about 10-15% end up committing suicide.