If u have a debt card that has only "plus" printed behind, u can only withdraw money form a "plus" network ATM overseas.
If your debut card has cirrus, than u can only withdraw money form a "cirrus" network ATM overseas.
This will work like taking a draft on a normal credit card.
However, it does not mean that ur DBS card will be able to withdraw money from a DBS ATM overseas. It depends on wether the ATM is "compatible" with your card. I experienced this problem when I was overseas. In the end, i had to go to a HSBC ATM just because the HSBC ATM is on the "cirrus" network while the DBS ATM was dont know on what network.
However, when u purchase things, the debit card will work the same way as a credit card. Just that your "credit limit" is what u have in your bank. And once a merchant brings a "charge" to your card, the bank will automatically "hold" the charged amount and u will not be touch it.