what were the causes of the Great Depression?

When discussing depression as a symptom, a feeling of hopelessness is the most often described sensation. Depression is a common psychiatric disorder in the modern world and a growing cause of concern for health agencies worldwide due to the high social and economic costs involved. Symptoms of depression, like the disorder itself, vary in degree of severity, and contribute to mild to severe mood disturbances. Mood disturbances may range from a sudden transitory decrease in motivation and concentration to gloomy moods and irritation, or to severe, chronic prostration.

There were different explanations for the causes of the Great Depression. But in simple, the causes can be summarized in a few points:

over-production and under-consumption;
bank failures;
stock market crash;
deflationary policies;
brokedown of international trade.
For more information, please read:
http://en.wikipedia.org/wiki/Great_Depression#Causes
http://en.wikipedia.org/wiki/Causes_of_the_Great_Depression