For almost 10 years, from 1929 to the late into the 1930's the word went into a massive economic decline. The entire world was hit with the decline, however the most industrialized nations were hit the hardest. These nations included the United States, Europe and Japan. Due to the nature of the depression economies that were built around industry were hit the hardest.
Although every industry was during these years, industries such as agriculture were hit less hard due to the fact that they remain basic necessities no matter what the economic situation of an economy is. This doesn't speak true however for industries such as construction which come to a complete halt when there is no economic activity.
The two main causes for the great depression was the vast unequal distribution of wealth during the 1920's as well as extensive stock market speculation that started to take prominence the the later 1920's. Imbalance in wealth creates a largely unstable economy, and this resulted in several parts of the world. During the 1920's because it was the biggest boom the United States economy had ever seen, the stock market, despite speculation, remained artificially high. Once the legs fell out from underneith the stock market, the unstable economy (created from the maldistribution of wealth) made the entire economy of America to collapse.
America right now looks to be headed in the same direction once again: the gap is growing between the rich and poor, and we are again in a major bust where we the majority of us are living beyond our means. Will history repeat itself? Are we smart enough to avoid another great depression? These are both questions that we will probably see answered in our lifetime.