Debit cards are a convenient way to access funds directly from a bank account. They can be used in physical stores by swiping the card at a point-of-sale terminal, which deducts the purchase amount directly from the user's bank account. Alternatively, debit cards can also be used for online shopping by entering the card details, such as the card number, expiration date, and security code, during the checkout process. This allows for secure transactions without the need for cash or credit.