Help me understand how credit card interest is calcualated?

[Thequestioner]

So lets say someone owed exactly ,000 on a credit card and had been paying minimum payments and raking up interest charges at 20%, and one day they came upon the money to pay the bill in full. The bill said you owe K exactly and the payment due date is December 20th. If you paid the ,000 bill in full by December 20th would the account be totally paid off? In other words does the credit card company calculate how much interest you would owe on the principal amount if you paid the bill by the due date 12/20, OR would you have to pay the interest that accrued during the billing cycle up to the date they received the money (December 20th) in the next bill in January?

[Glen]

Interest is a cost to a borrower for borrowing money. An interest rate is a way of expressing this for a set time-period, usually a year and is always expressed as a percentage of the amount borrowed.If you buy something with a credit card, you are borrowing money from your card company to pay for it. If you don’t repay the amount you owe, in full, by the payment due date, your card company will charge you interest. Do not confuse the interest rate with the annual percentage rate (APR).If you don’t pay the amount owing on your credit card in full, by the payment due date, your card company will charge you interest according to the type of transactions you have made.