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Cashier's checks are issued by the bank. They aren't personal funds, they're bank funds, so they're seen as more secure. When you buy a cashier's check/certified check, you aren't depositing money into an account, you're BUYING a check.
I would be more comfortable depositing a cashier's check than a personal check.
personal = straight out of the person's bank account
cashier's check = money is already accounted for so the check is guaranteed to have sufficient funds. in other words, it's paper money.
Technically, they aren't guaranteed...they're just much more safe. If the issuer doesn't have the funds, then you're out of luck just like a personal check...but the odds of a bank/issuer lacking funds is much more unlikely.