A simple way to categorize the different types of depression is to distinguish between a  (1) reactive or  (2) an endogenous depression.

(1) Reactive Depression
Many psychologists agree that 75% of all depressions are reactions to some external event.  A reactive depression is a response to some unpleasant occurrence such as the death of a spouse or a child, or not getting a promotion, not being accepted, to be helpless in a particular situation, a loss of some financial means, a sickness, having done something wrong.

This type of depression can last for a few weeks or months with ups and downs, but the person is still able to function normally most of the time.

(2) Endogenous Depression
This type of depression is not a response to an occurrence from the outside world but is initiated by some unknown internal process.  Endogenous depression is something that "swoops down" on the person.  It recurs periodically.

This type of depression may be further divided into two different categories:  (1) unipolar, and  (2) bipolar.

In the unipolar type, there are alternating moods of despair and neutrality.

How to deal with it
The bipolar  (or manic)  depression repeatedly cycles through despair, neutrality, and returns again to a state of despair, thus starting the entire cycle again.

Some psychologists are inclined to think that an endogenous depression has a psychological basis, and that it is transmitted in the genes.

Because everybody suffers sometime, somehow from depression, it is good to know what to do and how to prepare oneself before it appears.  To learn how to cope with it and to have some defense mechanism is not only helpful for the person himself, but it will enable him to help other people who are suffering from a depression.

If a person has suffered a loss, he will experience a reactive depression.  Allow him to grieve for the loss.  It always takes time and patience to digest it.  By recognizing and experiencing anger, sadness, and other strong emotions which are normal responses to a loss, a person may avoid the kind of serious deep depression which can occur in people who keep these strong emotions bottled up, who never show them, and who refuse to deal with them.