An simple Bank cheque is something you issue ,ie you write the beneficiary name ,amount to be paid ,date and signature.
But there is no guarantee that funds would be available when the beneficiary presents the cheque at the bank.Typically a cheque bounce scenario.
DD is a lot safer for the beneficiary as the amount payable is already collected by the bank and only then does the bank issue the DD.Now it is the bank that is liable to pay and banks are a lot more trust worthy than individuals.
Certified cheque is just what it says.It is certified by the bank and payment is guaranteed.
Read on if you want to know how this works :
You nrequest for a certified cheque.
Bank verifies that funds are avail in your account .
Bank lien marks the said amount (funds still in your account but you cannot see it as available funds and also you cannot spend/withdraw that amount.The rest of your funds are still available to you )
And bank guarantees payment to beneficiary as the lien marked funds in your account is controlled by bank.
hope this helps......