In the simplest terms, a collateralized debt obligation (CDOs) is a loan on something that has collateral. Unlike a simple loan at your local bank, CDOs have specific rules about who should invest in them, how they are rated and are specific in their scope of investment.
Local banks or credit unions need to free up money so they can loan again to local investors and borrowers. They will often sell the loans that they have on their books to another, larger firm. These providers will gather all these loans together, and slice them into pieces called tranches. These tranches are then sold to larger institutions such as hedge funds to carry in their portfolios.
CDOs are put together for a special purpose. They are a pool of assets of a particular kind such as high yield bonds or asset backed securities. Pooling them together helps the rating services rate them, and big, institutionalized investors know how risky they are when adding them to their portfolio
There are two basic types of CDOs: the synthetic CDO, which is issued to help banks or insurance companies to free up money on their books. The second is the Arbitrage Transaction, which allows the sponsor to earn a yield on the spread between the yield offered and the payments that are made to the various tranches.