Credit default swaps.
Or basically insurance for a lender.
Unfortunately, companies like Lehman and AIG ran these from a non-insurance arm of their business. Which, btw, are not regulated by the govt. So they had no or little reserves, and took in the premiums, if you will, at almost 100% profit, thinking they'd never have to pay out because as we all know, real estate always goes up. Except this time.
Oh, and if you are the issuer of a CDS, you can sell those too.
So, one mortgage may be tied to 2 or more CDS.
The CDS market is about $65,000,000,000,000. No, that is not too many zeroes. 65 TRILLION dollars are floating on CDSs. Took down Iceland and almost the rest of the world.
You cen bet there will be a slew of regulations world wide to control CDSs from here on out.