credit card system is a type of retail transaction settlement and credit system, named after the small plastic card issued to users of the system. A credit card is different from a debit card in that it does not remove money from the user's account after every transaction. In the case of credit cards, the issuer lends money to the consumer (or the user). It is also different from a charge card (though this name is sometimes used by the public to describe credit cards), which requires the balance to be paid in full each month. In contrast, a credit card allows the consumer to 'revolve' their balance, at the cost of having interest charged. Most credit cards are the same shape and size, as specified by the ISO 7810 standard.

How Credit Cards 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. The cardholder indicates their consent to pay, by signing a receipt with a record of the card details and indicating the amount to be paid or by entering a PIN. Also, many merchants now accept verbal authorizations via telephone and electronic authorization using the Internet, known as a customer not present (CNP) transaction.
Electronic verification systems allow merchants 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. The verification is performed using a credit card payment terminal or POS system with a communications link to the merchant's acquiring bank. Data from the card is obtained using from a magnetic stripe or chip on the card; the later system is commonly known as Chip and PIN, but is more technically an EMV card.
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. These will typically involve the cardholder providing additional information, such as the security code printed on the back of the card, or the address of the cardholder.
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.
For example, if a user had a $1,000 outstanding balance and pays it in full, there would be no interest charged. If, however, even $1.00 of the total balance remained unpaid, interest would be charged on the full $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.