Okay I'll try to explain the chart that IR posted:
Left hand side:
1) Lender provides cash to the borrower (they vet the borrower through a loan broker or directly). The borrower signs documents that explain the conditions of the loan with regards to interest rate, points (money paid at the beginning of the mortgage), prepayment penalties (money you pay for paying your loan off before a certain date), the term that the loan remains fixed if it is an adjustable rate product, and the index used to calculate payments. On an option mortgage you may be provided with the choice to make a) a minimum payment that is calculated based on the rate you would pay on a fully amortizing mortgage at a low teaser rate such as 1% b) a interest only payment: you are not paying down principal if you make this payment c) 30 year amortizing payment: you will pay off the loan in 30 years if you make this payment every month or d) 15 year amortizing payment: if you make this payment you will pay off your loan in 15 years.
2) The cash from the payments made by the borrowers goes to a servicer who is paid to deal with the borrowers and assign the payments to the correct lenders. Typically these pools are tranched so that means that there is an order of who gets paid. You can slice and dice the mortgage into any number of securities with fixed or variable coupons (interest payments are either the same each month or increase or decrease with interest rates). Generally the idea is to make securities with a high probability of payment so that banks, insurers, and other entities that are required to take low risk investments can purchase them. Even if the borrower is a 500 FICO the first 50% of payments may have a very high probability of payment and would be assigned a AAA rating. Generally you have multiple securities with multiple credit ratings so investors with different risk tolerances can invest in them. The equity tranche is paid last so it generally has a high interest rate but high uncertainty of payment. Usually the banks or nonbank entities that put the deal together hold a portion of the equity tranche. This is one reason why Merrill Lynch is taking such huge losses.
Relationship between Lenders and Banks:
As I stated banks and insurers tend to be big investors in mortgage backed securities. So they get cash flows as the borrowers make payments, the properties get refinanced (both return of capital and the prepayment penalties), and as properties are taken back as REO and sold. Generally you get less than the amount owed inclusive of legal expense for properties that are taken back as REO so you might get 70 cents on the face value + legal fees on these properties.
PMI and Lender's Insurance Policies:
The borrower or the lender can take out insurance to protect the lender against losses in excess of 80% of the original loan balance. This is often referred to as private mortgage insurance. Downey Savings has a policy protecting them against losses in excess of 20% on each asset in their portfolio. Whatever insurer wrote that policy is likely to make substantial payments to Downey.
Special Investment Vehicles:
Banks do not like holding assets on their balance sheets because they have to hold reserves against the probability of nonpayment on loans. So they set up SIVs to buy mortgage securities or credit default obligations off balance sheet then issue commercial paper. The risk is that the mortgage securities often had a longer duration 3-5 years than the commercial paper (30-180 days). There is also the risk that the securities may decline in value or payment may not be assured. In the current credit crunch there has been significant unease that some of these assets are worth less than their outstanding debt and some banks have had to either extend credit to their SIVs or move the assets back on to their balance sheets.
Relationship between SIVs and the Commercial Paper Market:
Generally companies and financial firms tend to invest their cash in commercial paper for short term cash needs. Commercial paper from SIVs and asset backed issues became substantially less liquid so companies were unable to access this money. So futures traders cash was tied up and couldn't be used for settlement and miners in Canada could not be fed. Generally Commercial Paper is viewed as relatively unrisky due to the short durations but some investors wanted to get higher returns by taking more risk like Fidelity and very likely Citicorp's Money Market funds. My understanding is the goal of the Super SIV may be to prevent Citicorp from breaking $1 on their money market funds. This would create losses for investors but the loss of face for Citicorp and loss of capital would be very damaging to them. From Citicorp's perspective it may be worth paying fees to assign a par value and place the assets into the Super SIV even if that makes no economic sense if it allows them to avoid breaking $1 on their money market mutual funds. The alternative is being forced to raise more bank capital which would likely come at a substantially greater cost than bank deposits.
Collateralized Debt Obligations to the Asset Backed Securities Market:
Collatalized Debt Obligations typically buy a portfolio of asset backed securities then make a new waterfall of tranched securities of varying credit risk and payment structure. The term waterfall is used because the payments flow down from the AAA security to AA securities and so on down to the equity securities. Think of a CDO as like an actively managed mutual fund of asset backed commercial paper. In theory the manager does due diligence of what the collateral is for each of the securities and the investors are paying a fee for that service. CDOs can issue AAA paper that is backed by a range of credit which may include equity pieces of subprime mortgage securities, car loans, home equity loans, GE commercial paper, etc.
Relationship between CDOs and Insurers:
You can insure a CDO just like an Asset Backed Security to increase the credit rating or credit profile of that offering or that particular security. In exchange for a fee the insurer will make payments in the event of a default on the underlying assets. Many hedge funds were buying the low rated offerings of the CDOs because they offered a high coupon rate. They may have felt that there was some arbitrage whereby the AAA buyers were paying too much premium for the certainty of payment. There was an argument amongst hedge funds that China and the oil producers were driving down AAA yields to values that were too low. They were partially right but neglected to realize that the underlying collateral of their CDO equity slice was a piggyback mortgage on a $1,000,000 houses with no cash down and 510 FICO borrower.