When you use a credit card to purchase something, you are making yourself a loan through the credit card company. You have to pay the company back for this loan at the terms you have agreed to when you signed the application for the card.
If you make a payment in full when you receive your monthly bill, there will be no additional amount due, no interest, and usually no handling fee. When you make a partial payment, whether it is the minimum due, or a larger amount, the company will charge interest, and perhaps a monthly fee, which will be added to the next monthly bill. As long as the amount you pay is less than the amount due, you will continue to be charged interest every month, based on the balance remaining. If you pay the entire amount due at the end of the month, there will be no new interest charges. There might be a small amount of interest on the previous balance. Often, if you call the company and point out that you paid the previous bill in full, they might waive the final interest due.