Depression is "temporary" imbalance of biochemicals in the brain. Now that we've said that. What does it mean?
While it is perfectly normal for people to go through periods of sadness, worthlessness, etc, for short periods of time, depression is a disease in which that state is extended, and lasts more than the norm; which varies from person to person. (About a week).
Depression however, is prolonged periods of feeling "down", worthless, sad, and hopeless. These periods can last anywhere from a month to a year, to -if untreated- a lifetime.
Although it is the most treatable of all mental illness, it is rarely proffesionally recognized, and most cases it goes untreated.
There is no one cause of depression, and it is not fully understood. However, some factors that are common in depression cases are:
-Specific distressing life events (trauma)
-Biochemical imbalance in the brain.
-Psychological factors such as pessimistic views of life.
Trauma can be anything from a car crash, to an exam failure after hours of study, to death in the family or friend circle.
Psychological factors from pessimism is just that. If you look at life negatively all the time, you will convince yourself that everything is hopeless, which will send anyone into depression.
Depression also may be cause by heredity. Some research has pointed to a link between cases in families where a reletive had depression and now another family member in a different generation has it too. There is information that points to a direction that it is more likely for you to get depression if there is a history of it in your family.