The Great Depression was a worldwide economic downturn starting in most places in 1929 and ending at different times in the 1930s or early 1940s for different countries. It was the largest and most important economic depression in the 20th century, and is used in the 21st century as an example of how far the world's economy can fall. The Great Depression originated in the United States; historians most often use as a starting date the stock market crash on October 29, 1929, known as Black Tuesday.
The depression had devastating effects in virtually every country, rich or poor. International trade plunged by half to two-thirds, as did personal income, tax revenue, prices and profits. Cities all around the world were hit hard, especially those dependent on heavy industry. Construction was virtually halted in many countries. Farming and rural areas suffered as crop prices fell by roughly 60 percent. Facing plummeting demand with few alternate sources of jobs, areas dependent on primary sector industries such as farming, mining and logging suffered the most.
The Great Depression ended at different times in different countries. The majority of countries set up relief programs, and most underwent some sort of political upheaval, pushing them to the left or right. In some states, the desperate citizens turned toward nationalist demagogues-the most infamous being Adolf Hitler-setting the stage for World War II in 1939.
A Shorter Answer
The 'Great Depression' was a period in the United States and world history when business was poor and many people were out of work. It was caused by many things like the drought that ruined so many farms and the stock market crash. Therefore, people lost money and jobs, because people had less money to spend or invest.