Most Amex cards are regular credit card accounts, which allow you to carry a balance on your account as long as you pay at least the minimum amount listed on your statement by the due date. If you do carry a balance, you'll be charged interest on it at the applicable rate.
With an Amex charge card – such as the American Express Platinum Card – you're required to pay off your balance in full for each statement period. As you don't carry a balance on these cards, standard interest rates do not apply. However, there can be serious penalties if you don't pay the full balance by the due date on each statement.