It's a form of cheque that is issued by a bank on your request - but the difference is that it is 'cleared funds' - i.e. when you ask a bank to issue a bankers draught you must already have the cash to cover it in your account as it will be deducted immediately, unlike a cheque which takes around 3 days to clear. Some people only accept cash or banker's draught as these methods of payment cannot 'bounce'
Its a Bank Draft and they do still take 3 days to clear through an account although the payee knows that the funds are guaranteed - unless the draft is counterfeit / stolen of course.