Go Banking Rates notes that the main differences between a money order and a cashier's check are how each is purchased and how much money each can be written for. A money order can be purchased at retail outlets, the post office and check-cashing services for a small fee. The money order is secured by an upfront cash payment. Cashier's checks can only be purchased at a bank.
About.com reports that a cashier's check is secured by the funds in a customer's account. The cashier's check is guaranteed by the bank to be valid. Cashier's checks can be used for large purchases, such as the down payment on a house or car. A money order may have a financial limit, such as $1,000, depending on the issuer.
Anyone with the cash for it can purchase a money order while a cashier's check may only be available from a customer's bank. Someone who does not have a bank account may not be able to purchase a cashier's check. My Bank Tracker notes that some banks may be willing to issue a cashier's check for a non-bank customer if the person has the full amount of the cashier's check available in cash. The bank may also charge a larger fee for issuing the cashier's check than it would for a bank customer.