What is the difference between a cashiers check and a certified check and a bank check?

There is no real difference, just different names for the same idea. A cashier's/bank/certified check is a check which a guarantor, normally a bank, has written in exchange for cash or immediate withdrawal from a depositor's account. The funds are collected in advance by the bank so the payee (to whom the check is payable) has guarantee that they will be paid when the check is properly negotiated.