The above is only applicable when your outstanding bill does not contain cash withdrawals made from ATMs or cash advances or similar programs. While it's easy to make a cash advance, it comes with a fee and is charged an interest from the date you make the withdrawal. This means that even if you pay in full by the due date, you will still be charged interest on that cash amount taken from the ATM.
Credit cards can benefit you if used responsibly. You can enjoy discounts and privileges like rebates, rewards points and travel miles for all your retail purchases to maximize the benefits of up to 21 days interest free. However, if you do not take charge of your credit, you may find yourself in debt quickly.