The idea in a CDO transaction is to securitize debt collateral to make it more attractive to different classes of investors. In the earlier days, CDOs purchased high yield/emerging market debt through a special purpose vehicle (SPV) and raised funds by issuing securities ranging from AAA to BB/B. The overall risk of the portfolio of various collateral is tranched from relatively safe to speculative to satisfy different degrees of risk appetite.
Beginning with the most senior class, the cash flows from the collateral are used to service the outstanding notes sequentially. Losses are allocated on the basis of reverse seniority. This basic structure is also known as a cash flow CDO because the collateral cash flows are used to sevice the outstanding securities. Every CDO has an asset side, generating its revenue, and a liability side, whose obligations need to be satisfied. The difference between them is termed the funding gap.