Although some people might use the terms "bank check" and "cashier's check" interchangeably, they're not the same thing. Banks issue many different kind of checks, with personal and cashier's checks among the most common. People use these different types of checks for different reasons.
Different Funding Sources
A personal check is written by you and draws the money from your checking account. The funds are backed by that account. If there aren't enough funds there, the check won't clear and the payment will be reversed. A cashier's check requires you to give the bank the cash, and the bank teller then generates the check for the amount you designate. When the recipient presents the check to her own bank, the funds are drawn against the bank's funds rather than your account.
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When Secure Funds Matter
A cashier's check is typically seen as a more secure payment than a bank check because it was purchased with cash. For this reason, some transactions might require a cashier's check as payment. When you're closing on a home purchase, for example, you'll often have to provide a cashier's check for the agreed-upon down payment. The downside is, banks usually charge a fee for cashier's checks. And because cashier's checks are less convenient than personal bank checks, the bank check is a better option for most purchases. Just make sure you have enough in your account to cover the purchase amount.