Questions of opening the bank?

In short,
1. Your debit card draws money you have in your CBA account to pay for your expenses electronically.
2. A credit card actually allows a consumer to borrow instant money from the bank to pay for the purchase. It will run up a debt for you. If you don't pay up at the end of a period, they'll charge you interest on the amount owed to the bank.

Here are some more points to fill in the missing information you listed in your inquiry
Q. I have a book account in Commonwealth bank.
A. That's how your expenses are covered. The bank will take money out of this account to pay for your purchases.
Q. Can I withdraw the money in ATM by Debit Mastercard?
A. As long as that ATM is mulitlinked to the CBA, you should be able to take money out, minus a transaction fee if it's not a Commonwealth Bank ATM.
Q. And I heard that international student cannot get the credit card in Australia, isn't it?
A. Foreign student or any body, without full time employment income and records, are usually not eligible to obtain for a credit card.

Some shops accept cash or Credit Card (such as AmExp, Visa, Mastercard) only.

Using a debit card, you can pay for goods and services with Credit Card System also. That means you swipe the card and then sign the docket to pay. There is no difference to Credit Card, when you process the transactions at shops.

The difference between Credit Card and Debit Card is, Using Credit Card means the bank or Card Authorization will pay goods or services for you first, then you pay back the bank. Using Debit Card means the money from your savings account will be transferred the shop's account. (but in debit card case, the savings accounts must have enough money to pay for your buying. )

Debit Card has an convenience that when you have no credit card, and the shop has no eftpos but only credit card system, you can use debit card to pay through credit card system.

There are basically two common types of bank card (as i would say ha) one is debit, the other is credit. When you swipe a debit card, you are using what you already have in your account (there are two separate accounts: saving and checking; you can take out money from both, but you can only take money out from checking if you swipe it ). and when you swipe a credit card, nothing get deduct from your account, however, you have to pay back the amount of money in a limited time and you should get a statement in your mail monthly.

My advice is to get a debit if you have monthly deposit, and because you don't have to worry about spending to much money without knowing it if you are a shopaholic you know what i mean ;) with that being said, it's also important to at least have one credit card to build up your credit for future references (buying cars/houses...)