I never used to like this part of American history, but working with piece of literature (Steinbeck, for example), I had to learn about the Depression it is something I find fascinating. Its causes were many: after WWI, people started to manufacture and export like crazy. Farmers invested in new machinery so they could produce more crops to ship overseas to countries devastated by the war. The U.S. made loans to other countries suffering the after-effects of the war.
The Stock Market was a mess because money was invested, but no one was keeping track of the number of stocks being sold, whether there was money to support the sale of those stocks; bank loans weren't secured. Cars were manufactured beyond demand—only to sit on the lots. The money was invested in the cars, borrowed from banks, but could not be turned into cash if no one was buying cars. When people got nervous and started to try to withdraw their savings (hello, It's a Wonderful Life), there was a run on the banks. Money in banks had been invested in loans to build homes and businesses. When everyone went to get their money at the same time, the banks had to close down—loans had been made, which meant the money wasn't in the bank, but invested in the community. (Interest collected on loans allowed banks to pay dividends to their customers.) The Stock Market plummeted, people lost jobs and homes. Farms failed and went to seed as banks foreclosed on properties. There was a terrible drought. People traveled across the country to find jobs. I read somewhere that four or five girls might share an apartment, having only two nice dresses between them. They would take turns wearing the dresses—only two girls going out each day—to see if they could get jobs so they could survive. People who traded heavily in the Stock Market lost everything. Families that had once been wealthy were quickly reduced to living very frugally if they were lucky, or finding themselves broke. There were soup and bread lines to keep people from starving. Some families lived in lean-tos, where a piece of metal (for example) was placed on an angle (leaning) against a building. This was all a family might have to protect them from the weather. It's hard to imagine how bad it was. I know my mother said they ate bread and butter for dinner and were happy because they weren't hungry. Things we take for granted today (chocolate, going to the movies) were luxuries people then could no longer afford...but it also showed what the American people were made of. They worked hard and pulled together to get through. In the 1930, FDR introduced programs to aid people and jump-start the economy. Then in the 1940s we were at war again. And wars mean jobs. Sadly, the economy didn't really turn around until the 1940s.