If you have a debit (aka “ATM”) card associated with your checking account chances are that it sports a MasterCard or Visa logo. You may use it to pay for purchases as if it is a credit card when the merchant does not use a debit keypad for entering your PIN to authenticate the transaction. You may also use it for most other transactions that require a credit card online.
This is very convenient, especially if you only want to spend real money in your checking account rather than accumulate debt on a credit card. But there’s a very good reason NOT to use your debit card as a credit card.