Interest charges

Credit card issuers usually waive interest charges if the balance is paid in full each month, but typically will charge full interest on the entire outstanding balance from the date of each purchase if the total balance is not paid.
For example, if a user had a $1,000 transaction and repaid it in full within this grace period, there would be no interest charged. If, however, even $1.00 of the total amount remained unpaid, interest would be charged on the $1,000 from the date of purchase until the payment is received. The precise manner in which interest is charged is usually detailed in a cardholder agreement which may be summarized on the back of the monthly statement. The general calculation formula most financial institutions use to determine the amount of interest to be charged is APR/100 x ADB/365 x number of days revolved. Take the Annual percentage rate (APR) and divide by 100 then multiply to the amount of the average daily balance (ADB) divided by 365 and then take this total and multiply by the total number of days the amount revolved before payment was made on the account. Financial institutions refer to interest charged back to the original time of the transaction and up to the time a payment was made, if not in full, as RRFC or residual retail finance charge. Thus after an amount has revolved and a payment has been made, the user of the card will still receive interest charges on their statement after paying the next statement in full (in fact the statement may only have a charge for interest that collected up until the date the full balance was paid...i.e. when the balance stopped revolving).[1]
The credit card may simply serve as a form of revolving credit, or it may become a complicated financial instrument with multiple balance segments each at a different interest rate, possibly with a single umbrella credit limit, or with separate credit limits applicable to the various balance segments. Usually this compartmentalization is the result of special incentive offers from the issuing bank, to encourage balance transfers from cards of other issuers. In the event that several interest rates apply to various balance segments, payment allocation is generally at the discretion of the issuing bank, and payments will therefore usually be allocated towards the lowest rate balances until paid in full before any money is paid towards higher rate balances. Interest rates can vary considerably from card to card, and the interest rate on a particular card may jump dramatically if the card user is late with a payment on that card or any other credit instrument, or even if the issuing bank decides to raise its revenue.

Article by: http://wikipedia.com