Great depression was a depression of economy in the US history that is considered to be the worst. The economic depression also took place in other nations across the world but it had varied timings, but in most of them its start was in 1929 and end was in 1940. This depression started from the United States. The real causes and reasons of this depression are still a matter of debate among economists, although most of them think and it is the popular belief that it was caused by the crash of the stock market at 29th of Ocotber in 1929 (is remembered as Black Tuesday) and it quickly spread worldwide. It was not caused by this one incident, but instead a combination of other conditions.
Most economists and historians believe that the stock market crash which occurred on black Tuesday was what caused the great depression. With the crash of stock market the faith of people also dropped in the economy and stock market. This was one of the major causes but should not be mistaken as the only cause.
In the early 1930s after the stock market crash, banks in the US were uninsured. Till the end of 1930 about nine thousand banks were closed and collapsed and the deposited money of people was simply lost. The surviving banks that were left were unsure of the economic situation and were concerned only for their own survival and stopped lending money. Due to this, individual persons and teams of investors were unable to arrange the enough money for business and investment etc.
As businesses and banks began to fall, the government of United States created the Smoot-Hawley Tariff in 1930 which was to help protect the American companies. This caused the imports taxes to go through the roof and it lead to less and less trade between America and other countries along with some economic retaliation from them.
There were some other reasons and causes as well, like various failed policies of to try make the conditions better and to avert the natural disaster. An example of such efforts was the Mississippi drought which was of such huge proportion that many people couldn't pay their taxes and debts and had to sell their farms for no profit and thus the US fell lower and lower into this economic disaster and also dragged most of the western world with it.