The Great Depression was the period of time directly following the crash on Wall Street in 1929. The stock market crashed meaning that share prices fell dramatically. The effect of this was universal, as it set off bankeruptcies in many companies and the United States recalled all its loans from other countries therefore spreading it worldwide. In short, it caused incredibly high levels of unemployment, devaluation of money and manufacturing dropped because of that. Many people lost their homes and it took many years for not only the economic situation to right itself but for many peoples lives to get back on track.
Another important effect was that it prompted the US to call in all her loans, including those owing to her from Germany. The US had lent Germany a great deal of money with which to pay reparations, and this loan call-in did nothing to improve the economic situation of a nation that had only just recovered from hyperinflation.