How They Work :-

A user is issued a credit card after an account has been approved by the credit provider (often a general bank, but sometimes a captive bank created to issue a particular brand of credit card, such as Wells Fargo or American Express Centurion Bank), with which the user will be able to make purchases from merchants accepting that credit card up to a pre-established credit limit.

When a purchase is made, the credit card user agrees to pay the card issuer. Originally the user would indicate their consent to pay, by signing a receipt with a record of the card details and indicating the amount to be paid, but many merchants now accept verbal authorizations via telephone and electronic authorization using the Internet.
Electronic verification systems allow merchants (using a strip of magnetized material on the card holding information in a similar manner to magnetic tape or a floppy disk) to verify that the card is valid and the credit card customer has sufficient credit to cover the purchase in a few seconds, allowing the verification to happen at time of purchase. Other variations of verification systems are used by eCommerce merchants to determine if the user's account is valid and able to accept the charge.

Each month, the credit card user is sent a statement indicating the purchases undertaken with the card, any outstanding fees, and the total amount owed. After receiving the statement, the cardholder may dispute any charges that he or she thinks are incorrect (see Fair Credit Billing Act for details of the US regulations). Otherwise, the cardholder must pay a defined minimum proportion of the bill by a due date, or may choose to pay a higher amount up to the entire amount owed. The credit provider charges interest on the amount owed (typically at a much higher rate than most other forms of debt). Some financial institutions can arrange for automatic payments to be deducted from the user's accounts.

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.

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, either to incent balance transfers from cards of other issuers, or to incent more spending on the part of the customer. 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. As the rates and terms vary, services have been set up allowing users to calculate savings available by switching cards, which can be considerable if there is a large outstanding balance.