A debit card is a small plastic card that you keep in your wallet. The card is associated with your bank account and with you by means of identifying information such as a long number on the front, a security number on the back, your name and your signature.
A debit card allows you to pay for goods in shops using an electronic card reader. It also allows you to withdraw money from your bank account using an ATM, or by going in to the bank and showing the cashier your card.
With a debit card, you can only withdraw money that is actually in your account. You may have an overdraft which enables you to withdraw additional money over and above your positive balance, but this will typically be associated with charges. If you make several withdrawals in a short space of time, your bank may choose to honor them and place you over your limit, incurring heftier charges.