It is interesting that 2 keywords which form the center of all the economic meltdown, remain a mystery to anyone not involved in investment banking. In this post, I will try to explain not only the meaning but also the functioning of these keywords in relation to the sub-prime mess.
CDO and CDS are the two instruments or vehicles created by investment banks to lower the risks involved in sub-prime lending. As the markets started to crack during the dotcom bust in 2000-01, people lost trust in the markets and started hoarding the money. As a result,Liquidity (the amount of money available in markets) started drying up which further pushed the markets down. if the markets are in bad condition then the fund flows to safe deposits with banks and government securities as they offer lower but safer returns. But as people start to save, they reduce their consumption putting pressure on industries and economy.

To counter this phenomenon, US government started reducing deposit rates and interest rates to lure people to borrow and consume more. Over a period of 3 years interest rates were reduced by 50%. This action boosted people to invest in homes and borrow heavily. The government also lowered the discretion for loan borrowing and allowed loans to be given without thorough paper work and checks. Here we take a break to understand the concept of credit score.
Credit Score -
Usually the credit worthiness of an individual is measured on a credit score. This credit score evolves out of payment history of the individual. 800 is the perfect score and 620 is the cutoff score for the loans which means anyone with a scoreless than 620 should not be granted loan.
And we are back -
To make maximum profits banks started lending at breakneck speed and soon were offering loans even to jobless people but at a slightly higher interest rate (usually +2%) to counter for their unemployment. These loans came to be known as Sub-prime loans and were offered to people with credit score of less than 620. On the hindsight, one can argue that sub-prime loans were much more riskier than Prime loans but at that time it seemed to be in vogue and the best idea ever devised.
Now, the banks and institutions making sub-prime loans were commercial banks and the mortgage institutions and investment banks were not in the picture. But the greedy investment banks can not stay away from lucrative opportunity for long. But they can not directly lend to the individuals so they had to create an instrument to get involved.
Voila, I-banks created CDO's (Collaterized debt obligation). They argued that if bank gives a sub-prime loan then there is small chance that the individual will default. But if bank gives sub-prime loans to 10,000 individuals then the risk of default reduces as not all individuals will default at the same time. This model reduces the chances of default and makes the loans safer. Using this model, I-banks bought mortgages and created pools of individual mortgages into CDO's and started selling pieces of these CDO's to financial institutions. These institutions received the installment payments from loan bearers in return and the rate of return was higher than the market. Since the CDO's were covered by properties and default chances were very small, these were granted AAA rating by Moody's and other rating organizations. AAA is the best rating for any bond in US.
So, the I-banks took the individual sub-prime loans off the balance sheet of mortgage companies and commercial lenders but again sold pieces of CDO's back to these banks. Uptill this point, I-banks were minting money without actually getting their hands dirty. They were buying a product from market, modifying it and selling it back to the market. They also retained some of these CDO's with them for investment.
But a wise ass went a step further. He thought that since their is a infinitesimal chance of CDO defaulting, what if we can start selling insurance against these CDO defaults. Since CDO's will noit default all the premium we earn will be profit. The idea seemed amazing and I-banks got busy creating a new instrument for this purpose. They came out with CDS (Credit default swap). This is an instrument that gurantees the buyer insurance against CDO default. So, I-banks started selling CDO's to the financial organizations and then also started insuring these CDO's.