When you use your debit card or credit card for online money transactions, you have to mention your card number, name of the cardholder, expiry date of the card and CVV number on the back side of debit and credit cards.
This means that you are providing all details through online.
It is easier to get the details of your account.
Even after putting CVV number, the system venerates one time pass word which is valid for limited minutes - say ten minutes and you have to key in the one time password. Afterwards, there may not be any use with such one time pass word.
In case you are using cheque book, the culprit should know your signature and style of signing the cheque. This is very difficult and the culprits do a lot of practice and then only they use such forged signatures.
Anyhow, while you are using debit cards for online money transactions, keep always average balance and not huge balance.