When you deposit money into your bank account, those funds become part of that bank's total deposits. The bank uses those funds to lend to other customers, collecting interest on those loans and thereby making a profit. But one issue banks always need to think about is that they can't predict, to an exact science, when you will come back and want your money. They can't loan out all of their money, because their customers will want to withdraw money at unknown rates and times.One way banks have addressed this uncertainty is to offer a financial investment called a certificate of deposit or CD. A CD is an agreement between a depositor and the bank that the depositor won't withdraw the money for a certain amount of time in return for earning interest on those funds....