A very good read:
Modern financial "products" and "instruments" are often highly complex and abstract, but the entire edifice can be distilled down to this: the system is based on the assumption that all risk can be hedged, and the difference between the initial position's yield/gain (i..e. placement of capital at risk for a gain) and the cost of hedging the risk of the wager to zero can be skimmed from the system risk-free.
That is the entire system in a nutshell, and we can immediately see the advantages of this system over traditional Capitalism, where risk can be hedged but never to zero, and the return is correlated to the risk taken on.
In modern finance, high-risk "investments" (wagers) with high returns can be taken on without worry because any and all risk can be hedged to zero, even in super high-risk wagers.
And since even high-risk positions can be seamlessly hedged to zero, then there is no reason not to borrow money to increase the size of your wagers: since you can't lose, then why not? Wagering in risk-free skimming with borrowed or leveraged money is simply rational.
Put these together and we see how a system based on risk-free skimming eventually leverages itself to the point that the slightest disruption can bring down the entire over-leveraged, over-extended system.
http://www.zerohedge.com/news/guest-post-collapse-our-corrupt-predatory-pathological-financial-system-necessary-and-positive