Credit Default Swaps (CDS)

AIG lost a few tens of billions in Credit Default Swaps. Here is an explanation in Wikipedia.

A credit default swap (CDS) is a contract between two counterparties, whereby the "buyer" or "fixed rate payer" pays periodic payments to the "seller" or "floating rate payer" in exchange for the right to a payoff if there is a default or "credit event" in respect of a third party or "reference entity".

If a credit event occurs, the typical contract either settles by delivery by the buyer to the seller of a (usually defaulted) debt obligation of the reference entity against a payment by the seller of the par value ("physical settlement") or the seller pays the buyer the difference between the par value and the market price of a specified debt obligation, typically determined in an auction ("cash settlement").

A credit default swap resembles an insurance policy, as it can be used by a debt holder to hedge, or insure against a default under the debt instrument. However, because there is no requirement to actually hold any asset or suffer a loss, a credit default swap can also be used for speculative purposes and is not generally considered insurance for regulatory purposes.

http://en.wikipedia.org/wiki/Credit_default_swap