there is one big differnce between them both the methods, one used the payment mode during performing on camapiagn and another one you can used after completing the camapaign, and you can paid the money.
There is one big differnce between both the techniques, one used the transaction method during executing on camapiagn and another one you can use after finishing the camapaign, and you can compensat the cash easily.
when you run your google adwords account . then you wil get the two option for hte payment .
If you follow the automatic .then you have need to give the credit card billing address . And you will charged once weekly or monthly and your add is moving with out any problem . But google provide the authority for that profile .
In the manual section after setting the ads group as you give the billing detail .your ads move in the market . and stop when your balance reached minimum . At this time you have need to topup your balance .
Prepay means you have to paid in advance money to Google account so that you can run your campaign or ads. Postpay means you have to pay total amount after every month. But there are many threshold for postpay account. So please read guidelines before starting.