Mortgage Pool

http://www.investopedia.com/terms/m/mortgage_pool.asp

Definition of 'Mortgage Pool'
A group of mortgages held in trust as collateral for the issuance of a mortgage-backed security. Some mortgage-backed securities issued by Fannie Mae, Freddie Mac and Ginnie Mae are known as "pools" themselves. These are the simplest form of mortgage-backed security. They are also known as "pass-throughs" and trade in the to-be-announced (TBA) forward market. 
 
 Investopedia explains 'Mortgage Pool'
Pass-throughs or pools are comprised of mortgages with close to the same maturity and interest rate. A pool of mortgages that backs a more complex mortgage-backed security or CDO, however, might consist of mortgages of more varying interest rates and characteristics. 

Mortgage-Backed Security (MBS)

Definition of 'Mortgage-Backed Security (MBS)'
A type of asset-backed security that is secured by a mortgage or collection of mortgages. These securities must also be grouped in one of the top two ratings as determined by a accredited credit rating agency, and usually pay periodic payments that are similar to coupon payments. Furthermore, the mortgage must have originated from a regulated and authorized financial institution.

Also known as a "mortgage-related security" or a "mortgage pass through." 
 
 
 Investopedia explains 'Mortgage-Backed Security (MBS)'
When you invest in a mortgage-backed security you are essentially lending money to a home buyer or business. An MBS is a way for a smaller regional bank to lend mortgages to its customers without having to worry about whether the customers have the assets to cover the loan. Instead, the bank acts as a middleman between the home buyer and the investment markets.

This type of security is also commonly used to redirect the interest and principal payments from the pool of mortgages to shareholders. These payments can be further broken down into different classes of securities, depending on the riskiness of different mortgages as they are classified under the MBS.