CDOs are larger financial products that institutions then sell on a secondary market. Each CDO may include credit card debt, mortgages, auto loans and corporate debt. They are considered collateralized because borrowers have promised to repay the debt within each part of the CDO.

When the housing bubble burst and subprime borrowers went into default at high rates, the CDO market went into a meltdown. This caused many investment banks to either go bankrupt or be bailed out by the government. Despite this, CDOs are still in use by investment banks today.

If the loans within a CDO are mortgage loans, the product is often referred to as a mortgage-backed security (MBS). If the mortgage loans in the CDO were made to borrowers with less than stellar credit or no credit history, they're called subprime mortgages.

Synthetic CDOs have been criticized for serving as a way of hiding short position of bets against the subprime mortgages from unsuspecting triple-A seeking investors, and contributing to the 2007-2009 financial crisis by amplifying the subprime mortgage housing bubble.