The Great Depression was a severe worldwide economic downturn that happened from about 1929 until 1939. It started with the crash of the Stock Market, and ended with the outbreak of World War II.
The Great Depression was made much worse and lasted longer due to the "New Deal" policies of President Franklin Roosevelt. He expanded the size of the federal government, starting a number of programs designed to put people to work. The growth of government, however, put a burden on private enterprise that made it harder to come out of the depression.