Everyone experiences some unhappiness. The sadness and grief people experience are usually appropriate and necessary. Sometimes these trial and tribulations can even present an opportunity for personal growth. However, when depression becomes long lasting and interferes with daily life, it may indicate a depressive disorder. A depressive disorder is distinguished from normal sadness by its severity, duration, and accompanying symptoms.

Depression is not a character flaw that people can 'just get over'. Most people who are depressed do not seek psychiatric help. They suffer needlessly. Depression is a treatable illness. If you suspect that you suffer from depression please consult your family physician, a psychologist, or psychiatrist for help.

Major  Depression
The overarching feature of major depression is abnormality. Its main features are persistant saddness, marked functional impairment, disabling physical symptoms, and disabling psychological symptoms that last at least 2 weeks.

        Mood Symptoms

• abnormal depressed mood - hopeless, discouraged, and empty
• abnormal loss of interest and pleasure mood - crippling lack of motivation and reduced capacity to experience pleasure
• abnormal irritable mood - often alienates loved ones

Physical Symptoms

• abnormal appetite - either eating too little or too much
• abnormal sleep - either difficulty falling asleep, frequent awakenings during the night or very early morning awakening or the opposite, excessive sleeping
• fatigue or loss of energy - profound, prominent and disabling
• agitation or slowing - either actual physical slowing of speech, movement and thinking or observable pacing and physical restlessness

Cognitive Symptoms

• abnormal self-reproach or inappropriate guilt - marked lowering of self-esteem and self-confidence with increased thoughts of pessimism, hopelessness, and helplessness or feelings of excessive and unreasonable guilt
• abnormal poor concentration or indecisiveness - becoming quickly mentally fatigued when asked to read, study, or solve complicated problems
• abnormal morbid thoughts of death (not just fear of dying) or suicide - hopelessness

Atypical  Depression
The symptoms of atypical depression include overeating, oversleeping, a general feeling of sadness,strong feelings of rejection, and laeden paralysis (a sensation of heaviness especially in the arms). The ingestion of carbohydrates cause an increase in serotonin in the brain so the preference for overeating carbohydrates reflects a type of self-medication for these sufferers. An atypical depressive may be able to enjoy pleasurable circumstances despite being unable to seek out such circumstances. This contrasts with the "typical" depressive, who generally has reduced appetite and insomnia, and who is often unable to find pleasure in anything. Patients with atypical depression are more apt to have shorter but more frequent episodes of depression and are also more likely to be passive aggressive and have an obsessive-compulsive personality. Some studies suggest that the MAO inhibitors like phenelzine are more affective drugs in treating this disorder than the SSRIs or tricyclics. Despite its name, atypical depression is the most common subtype of depression.

Chronic  Depression
Chronic, but mild depression is called dysthymia. Dysthymia shares most of the same symptoms with major depression but is less intense and last at least two years. People with chronic depression suffer from low energy, a general negativity, and a sense of dissatisfaction and hopelessness. They do not exhibit marked changes in mood or in daily functioning and suicidal thoughts are not usually present. It can be described as a general sadness that always seems to be with you.
When people with chronic depression also have episodes of major depression from time to time, the condition is known as double depression.

Grief
Grief is a healthy emotional response to loss. It has much in common with the symptoms of depression but usually only lasts between three and six months. After this period of grief, people begin to become reinvolved with life and "move past" it.
If the grief remains severe, however, it increases the risk for on-going depression. This severe persistent grieving is sometimes called complicated grief disorder and the sufferer should seek treatment.

Situational Depression
Published the first week of April 2007 in Archives of General Psychiatry.
http://archpsyc.ama-assn.org/cgi/content/short/64/4/433
The gist of this article is that major depressive disorder is likely overdiagnosed 25% of the time. The only 'exclusion' to the diagnosis in the DSM IV is 'bereavement' and doesn't take into account depressive symptoms that people have as a normal response to losses or traumas such as divorce, financial failure, and other such situational factors, and the article points out that lots of doctors just give depression checklists and don't bother to do a good taking of history to find out the cause of the symptoms. The actual study doesn't say this but some think their real implication is that medications won't fix a divorce, financial disaster, and those kinds of things. This isn't really news, lots of mental health professionals have known this all along.

Hysteroid Dysphoria
The hysteroid dysphoria is characterized by repeated episodes of depressed mood in response to feeling rejected, and a craving for sweets and chocolate.

Seasonal Affective Disorder (SAD)
SAD is described as depression during the fall or winter. This type of depression lifts in the spring or summer, and may be replaced by a manic phase. The majority of people who suffer from SAD experience fatigue, a tendency to overeat, and a tendancy to oversleep in winter. Some people, however, experience loss of appetite and sleeplessness instead.

Premenstrual Dysphoric Disorder
An estimated 3% to 8% of women have PDD. The symtoms are severe depression, irritability, and tension for a week or so before menstruation. These symptoms dissipate after the menstrual period begins.