The truth is, it wasn't easy.
But one of the best 4 minute explanations ever is on this YouTube video: Causes of the Great Depression.
John Maynard Keynes, the king of Keynesian economics, would call these expansions and contractions, not recessions. You get them free with a capitalist economic system. With the exception of China, it seems that you may only get the contractions in communist systems (like USSR, Cuba, S. Korea and Venezuela).
Read more on the Great Depression at Wikipedia. As it pertains specifically to the USA, it is pretty heavy reading, though.
You can look at the similarities of the recessions of 2000 (the DotCom bomb) and the Great Recession of 2007-200x. In all cases there were financial bubbles at work. But the Great Recession was bubble-bulging in housing and financial markets throughout the USA and beyond. It effected all US industries and and all US States. No place to run from it, and no place to hide from it.
Apply called The Great Recession, it is a generational recession. That is, economists argue that you should only see such a recession about once in your lifetime. Note the massive overhang of shadow banking and the increase in uncertainty (including the use of derivatives).
Of course, you should only experience a hurricane about once in your lifetime or see a massive flood about every 500 years. Sometimes historical precedent does not accurately foretell the future?
You should expect markets to overshoot, maybe wildly, in the future. The overshoot will be to down side and to the up, as well.
Keep going up, but carry a parachute.
BTW. Check out this article about doing the same-old, same old, after a recession obviously suggests that a new approach is needed. Creating the same college degrees as if there would be jobs for them is, well, not smart!
Keywords: recession, Recovery, Great Depression, Great Recession, Keynes, Sustainable Education