"Securitization, in its most basic form, is a method of selling assets. Rather than selling those assets "whole", the assets are combined into a pool, and then that pool is split into shares. Those shares are sold to investors who share the risk and reward of the performance of those assets. It can be viewed as being similar to a corporation selling, or "spinning off," a profitable business unit into a separate entity. They trade their ownership of that unit, and all the profit and loss that might come in the future, for cash right now.
A very basic example would be as follows. XYZ Bank loans 10 people $100,000 a piece, which they will use to buy homes. XYZ has invested in the success and/or failure of those 10 home buyers- if the buyers make their payments and pay off the loans, XYZ makes a profit. Looking at it another way, XYZ has taken the risk that some borrowers won't repay the loan. In exchange for taking that risk, the borrowers pay XYZ interest on the money they borrow.
From the perspective of XYZ, those loans are 10 different assets. They have value- one, if the loan fails, XYZ takes ownership of the house. Two, if the loan succeeds, XYZ gets their money back along with the interest they charge.
XYZ can do two things with those loans. They can hold them for 30 years and, they would hope, make a profit on their investment. Or they could sell them to some other investor, and walk away. In doing this, they would make less profit than if they held onto them long term, but they would benefit in that they make some profit while also getting their original investment back. They give up some of the reward (profit) in exchange for not having the risk.
So XYZ Bank decides they'd rather have the cash now. They could sell those 10 loans to 10 investors. Each investor would be taking a risk in buying those loans, because if any loan defaults, that one investor loses. Naturally, investors would not be willing to pay very much for those loans, knowing the risk involved. XYZ wants to sell those loans for the best price they can get, so they decide to securitize those loans. They combine the 10 loans into one entity, and then they split that one entity into 10 equal shares. Each investor still pays the same $100,000, but instead of owning one loan, they will own 10% of all 10 loans. If one loan fails, every investor loses 10%.
The result is that XYZ bank is able to sell their assets for more money, and investors are insulated from the volatility of directly owning mortgages." from: http://en.wikipedia.org/wiki/Securitization