When I found Paddy Hirsch's video regarding Credit Default Swaps, I thought it was downright amazing. My colleague, Brad, had some solid feedback in his comment.
So I will attempt to tie-in Brad's comment and Paddy Hirsch's video(s)...
"CDOs (collateralized debt obligations) and repos (repurchase agreements) make up what economists call the "Shadow Banking System"...
CDO's
Repurchase Agreements
Shadow Banking
...this system allowed banks to become over-leveraged and overly-interconnected...
Leverage
Banks Interconnected
Thus, when the collateral in the CDOs -- in 2008, these were typically based in real-estate -- began to devalue, nearly every bank and financial institution imploded."

Numerous thoughts come to mind about this whole scenario. But the thought that sticks out the most is Hyman Minsky's Financial Instability Hypothesis which I have written about before (you can read it here).
I hope viewing Paddy Hirsch's videos coupled with Brad's comment (mixed with a shot of Hyman Minsky) explains this scenario well and doesn't leave you, badly, needing a drink.
;-)