This is an excellent loan for those that are lacking the down payment required for other types of mortgages.
The 80 20 mortgage is simply two loans for 100% of the purchase price. It is a first mortgage at 80% of the purchase price with a 20% second mortgage.
If you are a conforming borrower, doing your loan in this manner will save you from having to pay mortgage insurance. Mortgage insurance is almost always required when you have less than 20% down. But with the 80 20 loan you avoid this necessary evil.
If you are a sub-prime borrower, doing you loan in this manner will typically keep your interest rates