Some economists regard the 1929 stock market crash as major contributing factor to the great depression. The speculative boom of the 1920's caused the crash because of the build up of the economic bubble. The bubble was formed because in the 1920s, as the stock prices were increasing, many people invested in the market. As the prices kept increasing they continued to invest hoping the prices would go up forever. Most people borrowed money to invest in the market.
This continued till about 1929. Then the market started trading down. Most people panicked and this resulted in heavy selling of stocks. By the year 1933, the stock prices were down 80% from the highs in 1929.
This led to people feeling poor. This led to decrease in the demand for various products in the market. Companies that tried to raise money in the market failed miserably. This led to shortage of money for manufacturing products or providing services. Companies started firing their employees because they wanted to scale down production. As you can guess, this led to the great depression. This period lasted about 4-5 years till 1934. All this was caused due to lack in confidence. This was preceded by confidence in the stock market. This turn of confidence was caused by a small negative sentiment in the market.
The speculative boom of the 1920's was one of the factors that contributed towards the great depression. The speculative boom was caused due to the heavy investing in the market. The heavy investing was taking place due to most people trading on margin. Some traders were trading on 90% margin. The banks were also invested in the stock market. When the stock prices went down, people lost faith in the entire financial system and this lead to banks failing by the hundreds. This could have been avoided if there were proper regulatory procedures for the banks and the stock market in place. There should have been a limit on the margin you can use to trade. There should have been some restrictions on the banks from investing the depositors' money in the stock market.
Needless to say, the regulators learnt a lot from this cash. It required some time before the trust in the financial system came back. The federal government then set up the federal deposit insurance corporation. Due to the presence of FDIC the banks could run out of money to pay back but still escape as the government reimbursed the depositors. The regulatory rules and procedures in place now are stricter and prevent the economy from crashing like it did in 1929.
You as an investor or a trader can learn a lot from this crash. In the late 1920's people began to invest without doing any research about the stocks they were buying. In those times, the trader who was in the floor had more information than the common people trading. This led to lack of information among investors. Now, due to internet and disclosure policies, the common investor can have all the information about a company before investing in it. Good research will give you confidence about your investment and you will not panic when your stock price goes down or the general market conditions are bad.