What Is The APR On A Card?

Annual Percentage Rate is the interest rate which reflects the total annual cost on the interest of the loan, expressed as a percentage rate. Interest rates on credit cards change over time. Some of these adjustments are tied to changes in other interest rates like the Prime Rate or the Treasury bill rates, and are called the variable-rate plans. Others which are not tied to changes in other interest rates are called fixed-rate plans.
Fixed Rate: It is a fixed annual percentage rate of the finance charge. The finance charge is the dollar amount one pays to use credit.
Variable Rate: It is the Prime Rate (variable) plus added percentage. For instance, a certain credit rate may have a variable rate of PR+3.9(constant margin).

'Introductory rate' is another term which relates to a temporary lower APR that generally lasts for about  6 months before being converted to a normal, fixed or variable rate.
The Truth Lending Act which was passed in the year 1969, had strictly stated that the lenders should report their APR in the very first page of their loan contracts.