A cashier's check and a regular check are similar but they are different from each other.
With a regular check the funds come out of the checking account of the person who wrote the check.
With a cashier's check the person pays the bank the money to get the cashier's check and then funds for the cashier's check comes out of the banks funds.
A cashier's check is like a money order which guarantees the funds are available on the check written.