what is the difference between atm card and debit card

The main difference is in functionality.

An ATM card is primarily limited to cash withdrawals and checking account balances at an Automated Teller Machine (ATM).
A debit card offers all the functions of an ATM card but also allows you to make purchases in stores (Point of Sale) and transact online, as it is linked to a payment network like Visa or Mastercard.
In India, most banks now issue a debit card which effectively serves both purposes.